% egrep  '^(From|Subject|Date): '  mailbox
From: elvis@tabloid.org (The King)
Subject: be seein' ya around
Date: Mon, 23 Oct 2006 11:04:13
From: The Prez <president@whitehouse.gov>
Date: Wed, 25 Oct 2006 8:36:24
Subject: now, about your vote...

% egrep -i  '^(From|Subject|Date): '  mailbox

SUBJECT: MAKE MONEY FAST

% egrep -i '\<([a-z]+) +\1\>' files...

http://hostname/path.html

$/ = ".\n";
while (<>) {
  next if !s/\b([a-z]+)((?:\s|<[^>]+>)+)(\1\b)/\e[7m$1\e[m$2\e[7m$3\e[m/ig;
  s/^(?:[^\e]*\n)+//mg;   # Remove any unmarked lines.
  s/^/$ARGV: /mg;      # Ensure lines begin with filename.
  print;
}

$celsius = 30;
$fahrenheit = ($celsius * 9 / 5) + 32;  # calculate Fahrenheit
print "$celsius C is $fahrenheit F.\n";# report both temperatures

$celsius = 20;
while ($celsius <= 45)
{
  $fahrenheit = ($celsius * 9 / 5) + 32; # calculate Fahrenheit
  print "$celsius C is $fahrenheit F.\n";
  $celsius = $celsius + 5;
}

if ($reply =~ m/^[0-9]+$/) {
    print "only digits\n";
} else {
    print "not only digits\n";
}

if ($reply =~ m/^[0-9]+$/)

print "Enter a temperature in Celsius:\n";
$celsius = <STDIN>; # this reads one line from the user
chomp($celsius);   # this removes the ending newline from $celsius

if ($celsius =~ m/^[0-9]+$/) {
    $fahrenheit = ($celsius * 9 / 5) + 32; # calculate Fahrenheit
    print "$celsius C is $fahrenheit F\n";
} else {
    print "Expecting a number, so I don't understand \"$celsius\".\n";
}

printf "%.2f C is %.2f F\n", $celsius, $fahrenheit;

if ($celsius =~ m/^[-+]?[0-9]+(\.[0-9]*)?$/)  {

$celsius =~ m/^[-+]?[0-9]+[CF]$/
$celsius =~ m/^([-+]?[0-9]+)([CF])$/

print "Enter a temperature (e.g., 32F, 100C):\n";
$input = <STDIN>; # This reads one line from the user.
chomp($input);    # This removes the ending newline from $input.

if ($input =~ m/^([-+]?[0-9]+)([CF])$/)
{
    # If we get in here, we had a match. $1 is the number, $2 is "C" or "F".
    $InputNum = $1;  # Save to named variables to make the ...
    $type     = $2;  # ... rest of the program easier to read.

    if ($type eq "C") {      # `eq' tests if two strings are equal
        # The input was Celsius, so calculate Fahrenheit
        $celsius = $InputNum;
        $fahrenheit = ($celsius * 9 / 5) + 32;
    } else {
        # If not "C", it must be an "F", so calculate Celsius
        $fahrenheit = $InputNum;
        $celsius = ($fahrenheit - 32) * 5 / 9;
    }
    # At this point we have both temperatures, so display the results:
    printf "%.2f C is %.2f F\n", $celsius, $fahrenheit;
} else {
    # The initial regex did not match, so issue a warning.
    print "Expecting a number followed by \"C\" or \"F\",\n";
    print "so I don't understand \"$input\".\n";
}

if ($input =~ m/^([-+]?[0-9]+(\.[0-9]*)?)([CF])$/)

if ($input =~ m/^([-+]?[0-9]+(?:\.[0-9]*)?)([CF])$/)

if ($input =~ m/^([-+]?[0-9]+(\.[0-9]*)?) *([CF])$/)

$input =~ m/^([-+]?[0-9]+(\.[0-9]*)?)\s*([CF])$/

$input =~ m/^([-+]?[0-9]+(\.[0-9]*)?)\s*([CF])$/i

if ($input =~ m/^([-+]?[0-9]+(\.[0-9]*)?)\s*([CF])$/i)
{
 
    $type = $3;  # save to a named variable to make rest of program more readable

    if ($type eq "C") { # `eq' tests if two strings are equal
 
    } else {

if ($type eq "C" or $type eq "c") {

if ($type =~ m/c/i) {

print "Enter a temperature (e.g., 32F, 100C):\n";
$input = <STDIN>; # This reads one line from the user.
chomp($input);    # This removes the ending newline from $input.

if ($input =~ m/^([-+]?[0-9]+(\.[0-9]*)?)\s*([CF])$/i)
{
    # If we get in here, we had a match. $1 is the number, $3 is "C" or "F".
    $InputNum = $1;  # Save to named variables to make the ...
    $type     = $3;  # ... rest of the program easier to read.

    if ($type =~ m/c/i) {    # Is it "c" or "C"?
        # The input was Celsius, so calculate Fahrenheit
        $celsius = $InputNum;
        $fahrenheit = ($celsius * 9 / 5) + 32;
    } else {
        # If not "C", it must be an "F", so calculate Celsius
        $fahrenheit = $InputNum;
        $celsius = ($fahrenheit - 32) * 5 / 9;
    }
    # At this point we have both temperatures, so display the results:
    printf "%.2f C is %.2f F\n", $celsius, $fahrenheit;
} else {
    # The initial regex did not match, so issue a warning.
    print "Expecting a number followed by \"C\" or \"F\",\n";
    print "so I don't understand \"$input\".\n";
}

$var =~ s/Jeff/Jeffrey/;

$var =~ s/\bJeff\b/Jeffrey/;

$var =~ s/\bJeff\b/Jeff/i;

Dear =FIRST=,
You have been chosen to win a brand new =TRINKET=! Free!
Could you use another =TRINKET= in the =FAMILY= household?
Yes =SUCKER=, I bet you could! Just respond by.....

$given = "Tom";
$family = "Cruise";
$wunderprize = "100% genuine faux diamond";

$letter =~ s/=FIRST=/$given/g;
$letter =~ s/=FAMILY=/$family/g;
$letter =~ s/=SUCKER=/$given $family/g;
$letter =~ s/=TRINKET=/fabulous $wunderprize/g;

$price =~ s/(\.\d\d[1-9]?)\d*/$1/

From elvis Thu Feb 29 11:15 2007
Received: from elvis@localhost by tabloid.org (8.11.3) id KA8CMY
Received: from tabloid.org by gateway.net (8.12.5/2) id N8XBK
To: jfriedl@regex.info (Jeffrey Friedl)
From: elvis@tabloid.org (The King)
Date: Thu, Feb 29 2007 11:15
Message-Id: <2007022939939.KA8CMY@tabloid.org>
Subject: Be seein' ya around
Reply-To: elvis@hh.tabloid.org
X-Mailer: Madam Zelda's Psychic Orb [version 3.7 PL92]

Sorry I haven't been around lately. A few years back I checked
into that ole heartbreak hotel in the sky, ifyaknowwhatImean.
The Duke says "hi".
        Elvis

To: elvis@hh.tabloid.org (The King)
From: jfriedl@regex.info (Jeffrey Friedl)
Subject: Re: Be seein' ya around

On Thu, Feb 29 2007 11:15 The King wrote:
|> Sorry I haven't been around lately. A few years back I checked
|> into that ole heartbreak hotel in the sky, ifyaknowwhatImean.
|> The Duke says "hi".
|>         Elvis

while ($line = <>) {
    ... work with $line here ...
}

# Process the header
while ($line = <>) {
   if ($line =~ m/^\s*$/) {
       last; # stop processing within this while loop, continue below
   }
   ... process header line here ...
}
... processing for the rest of the message follows ...

if ($line =~ m/^Subject: (.*)/i) {
    $subject = $1;
}

if ($line =~ m/^Date: (.*)/i) {
   $date = $1;
}
if ($line =~ m/^Reply-To: (.*)/i) {
   $reply_address = $1;
}

From: elvis@tabloid.org (The King)

if ($line =~ m/^From: (\S+) \(([^()]*)\)/i) {
    $reply_address = $1;
    $from_name = $2;
}

while ($line = <>)
{

    if ($line =~ m/^\s*$/ ) { # If we have an empty line...
        last; # this immediately ends the `while' loop.
    }

    if ($line =~ m/^Subject: (.*)/i) {
        $subject = $1;
    }

    if ($line =~ m/^Date: (.*)/i) {
        $date = $1;
    }

    if ($line =~ m/^Reply-To: (\S+)/i) {
        $reply_address = $1;
    }

    if ($line =~ m/^From: (\S+) \(([^()]*)\)/i) {
        $reply_address = $1;
        $from_name = $2;
    }

}

print "To: $reply_address ($from_name)\n";
print "From: jfriedl\@regex.info (Jeffrey Friedl)\n";
print "Subject: Re: $subject\n";
print "\n" ; # blank line to separate the header from message body.

print "On $date $from_name wrote:\n";

while ($line = <>) {
    print "|> $line";
}

$line =~ s/^/|> /;
print $line;

if (    not defined($reply_address)
     or not defined($from_name)
     or not defined($subject)
     or not defined($date) )
{
    die "couldn't glean the required information!";
}

print "The US population is $pop\n";

... by Jeffrey Friedl.

... by Jeffrey Friedl.

... by Jeffrey Friedl.

... by Thomas Jefferson

s/\bJeff(?=s\b)/Jeff'/g

s/(?<=\bJeff)(?=s\b)/'/g

$pop =~ s/(?<=\d)(?=(\d\d\d)+$)/,/g;
print "The US population is $pop\n";

$text = "The population of 298444215 is growing";
   
$text =~ s/(?<=\d)(?=(\d\d\d)+$)/,/g;
print "$text\n";

$text =~ s/(?<=\d)(?=(\d\d\d)+(?!\d))/,/g;

$text =~ s/(\d)(?=(\d\d\d)+(?!\d))/$1,/g;

$text =~ s/(\d)((\d\d\d)+\b)/$1,$2/g;

while ( $text =~ s/(\d)((\d\d\d)+\b)/$1,$2/g ) {
   # Nothing to do inside the body of the while -- we merely want to reapply the regex until it fails
}

undef $/;    # Enter "file-slurp" mode.
$text = <>; # Slurp up the first file given on the command line.

This is a sample file.
It has three lines.
That's all

This is a sample file.
It has three lines.
That's all

This is a sample file.
It has three lines.
That's all

$text =~ s/&/&/g; # Make the basic HTML . . . 
$text =~ s/</&lt;/g;  #  . . . characters &, <, and > . . . 
$text =~ s/>/&gt;/g;  #  . . . HTML safe.

$text =~ s/^$/<p>/g;

$text =~ s/^$/<p>/mg;

... with.


	 
Therefore ...

$text =~ s/^[ \t\r]*$/<p>/mg;

... with.
<p>
<p>
<p>
Therefore ...

$text =~ s/^\s*$/<p>/mg;

... with.
<p>
Therefore ...

$text =~ s/\b(usernameregex\@hostnameregex)\b/<a href="mailto:$1">$1<\/a>/g;

$text =~ s{\b(usernameregex\@hostnameregex)\b}{<a href="mailto:$1">$1</a>}gi;

$text =~ s{
   \b
   # Capture the address to $1 . . . 
   (
     usernameregex
     \@
     hostnameregex
   )
   \b
}{<a href="mailto:$1">$1</a>}gix;

undef $/;    # Enter "file-slurp" mode.
$text = <>; # Slurp up the first file given on the command line.

$text =~ s/&/&amp;/g;      # Make the basic HTML . . . 
$text =~ s/</&lt;/g;  #  . . . characters &, <, and > . . . 
$text =~ s/>/&gt;/g;  #  . . . HTML safe.

$text =~ s/^\s*$/<p>/mg; # Separate paragraphs.

# Turn email addresses into links . . . 
$text =~ s{
   \b
   # Capture the address to $1 . . . 
   (
     \w[-.\w]*                                    # username
     \@
     [-a-z0-9]+(\.[-a-z0-9]+)*\.(com|edu|info)  # hostname
   )
   \b
}{<a href="mailto:$1">$1</a>}gix;

print $text; # Finally, display the HTML-ized text.

$text =~ s{
   \b
   # Capture the URL to $1 . . . 
   (
      http:// hostname
      (
         / path
      )?
   )
}{<a href="$1">$1</a>}gix;

$text =~ s{
   \b
   # Capture the URL to $1 . . . 
   (
      http:// [-a-z0-9]+(\.[-a-z0-9]+)*\.(com|edu|info) \b # hostname
      (
         / [-a-z0-9_:\@&?=+,.!/~*'%\$]*  # optional path
      )?
   )
}{<a href="$1">$1</a>}gix;

undef $/;    # Enter "file-slurp" mode
$text = <>; # Slurp up the first file given on the command line.

$text =~ s/&/&amp;/g;      # Make the basic HTML . . . 
$text =~ s/</&lt;/g;  #  . . . characters &, <, and > . . . 
$text =~ s/>/&gt;/g;  #  . . . HTML safe.

$text =~ s/^\s*$/<p>/mg; # Separate paragraphs.

# Turn email addresses into links . . . 
$text =~ s{
   \b
   # Capture the address to $1 . . . 
   (
     \w[-.\w]*                                    # username
     \@
     [-a-z0-9]+(\.[-a-z0-9]+)*\.(com|edu|info)  # hostname
   )
   \b
}{<a href="mailto:$1">$1</a>}gix;

# Turn HTTP URLs into links . . . 
$text =~ s{
   \b
   # Capture the URL to $1 . . . 
   (
      http:// [-a-z0-9]+(\.[-a-z0-9]+)*\.(com|edu|info) \b   # hostname
      (
         / [-a-z0-9_:\@&?=+,.!/~*'%\$]* # Optional path
           (?<![.,?!])    # Not allowed to end with [.,?!]
      )?
   )
}{<a href="$1">$1</a>}gix;

print $text; # Finally, display the HTML-ized text.

$HostnameRegex = qr/[-a-z0-9]+(\.[-a-z0-9]+)*\.(com|edu|info)/i;

# Turn email addresses into links . . . 
$text =~ s{
   \b
   # Capture the address to $1 . . . 
   (
     \w[-.\w]*                          # username
     \@
     $HostnameRegex  # hostname
   )
   \b
}{<a href="mailto:$1">$1</a>}gix;

# Turn HTTP URLs into links . . . 
$text =~ s{
   \b
   # Capture the URL to $1 . . . 
   (
      http:// $HostnameRegex \b          # hostname
      (
         / [-a-z0-9_:\@&?=+,.!/~*'%\$]* # Optional path
           (?<![.,?!])    # not allowed to end with [.,?!]
      )?
   )
}{<a href="$1">$1</a>}gix;

$/ = ".\n";
while (<>) {
  next if !s/\b([a-z]+)((?:\s|<[^>]+>)+)(\1\b)/\e[7m$1\e[m$2\e[7m$3\e[m/ig;
  s/^(?:[^\e]*\n)+//mg;   # Remove any unmarked lines.
  s/^/$ARGV: /mg;      # Ensure lines begin with filename.
  print;
}

% perl -w FindDbl ch01.txt
ch01.txt: check for doubled words (such as thisthis thisthis), a common problem with
ch01.txt: * Find doubled words despite capitalization differences, such as with `TheThe
ch01.txt: thethe...', as well as allow differing amounts of whitespace (space, tabs,
ch01.txt: /\<(1,000,000|million|thousandthousand thousandthousand)/. But alternation can't be
ch01.txt: of this chapter. If you knew thethe thethe specific doubled word to find (such

$/ = ".\n";   # Sets a special ``chunk-mode''; chunks end with a period-newline combination

while (<>) 
{
    next unless s{# (regex starts here)

          ### Need to match one word:
          \b           # Start of word . . . .
          ( [a-z]+ )  # Grab word, filling $1 (and \1).

          ### Now need to allow any number of spaces and/or <TAGS>
          (         # Save what intervenes to $2.
              (?:   # (Non-capturing parens for grouping the alternation)
                 \s       # Whitespace (includes newline, which is good).
                |         # -or-
                 <[^>]+>  # Item like <TAG>.
              )+       # Need at least one of the above, but allow more.
          )

          ### Now match the first word again:
          (\1\b)     # \b ensures not embedded. This copy saved to $3.

      #(regex ends here)
    }
    # Above is the regex. The replacement string is below, followed by the modifiers, /i, /g, and /x
    {\e[7m$1\e[m$2\e[7m$3\e[m}igx;  

    s/^(?:[^\e]*\n)+//mg;     # Remove any unmarked lines.
    s/^/$ARGV: /mg;         # Ensure lines begin with filename.
    print;
}

import java.io.*;
import java.util.regex.Pattern;
import java.util.regex.Matcher;

public class TwoWord
{
 public static void main(String [] args)
 {
   Pattern regex1 = Pattern.compile(
       "\\b([a-z]+)((?:\\s|\\<[^>]+\\>)+)(\\1\\b)",
       Pattern.CASE_INSENSITIVE);
   String replace1 = "\033[7m$1\033[m$2\033[7m$3\033[m";
   Pattern regex2 = Pattern.compile("^(?:[^\\e]*\\n)+", Pattern.MULTILINE);
   Pattern regex3 = Pattern.compile("^([^\\n]+)", Pattern.MULTILINE);

   // For each command-line argument....
   for (int i = 0; i < args.length; i++)
   {
     try {
       BufferedReader in = new BufferedReader(new FileReader(args[i]));
       String text;

       // For each paragraph of each file.....
       while ((text = getPara(in)) != null)
       {
           // Apply the three substitutions
           text = regex1.matcher(text).replaceAll(replace1);
           text = regex2.matcher(text).replaceAll("");
           text = regex3.matcher(text).replaceAll(args[i] + ": $1");

           // Display results
           System.out.print(text);
       }
     } catch (IOException e) {
       System.err.println("can't read ["+args[i]+"]: " + e.getMessage());
     }
   }
 }

 // Routine to read next "paragraph" and return as a string
 static String getPara(BufferedReader in) throws java.io.IOException
 {
   StringBuffer buf = new StringBuffer();
   String line;

   while ((line = in.readLine()) != null &&
          (buf.length() == 0 || line.length() != 0))
   {
       buf.append(line + "\n");
   }
   return  buf.length() == 0 ? null : buf.toString();
 }
}

if ($line =~ m/^Subject: (.*)/i) {
    $subject = $1;
}

    import java.util.regex.*; // Make regex classes easily available
        
Pattern r = Pattern.compile("^Subject: (.*)", Pattern.CASE_INSENSITIVE);
Matcher m = r.matcher(line);
if (m.find()) {
    subject = m.group(1);
 }

if (! Pattern.matches("\\s*", line))
{
    //  . . . line is not blank . . . 
}

if (! line.matches("\\s*", ))
{
    //  . . . line is not blank . . . 
}

Imports System.Text.RegularExpressions   ' Make regex classes easily available
   
Dim R as Regex = New Regex("^Subject: (.*)", RegexOptions.IgnoreCase)
Dim M as Match = R.Match(line)
If M.Success
    subject = M.Groups(1).Value
End If

If Not Regex.IsMatch(Line, "^\s*$") Then
   '  . . . line is not blank . . . 
End If

if (preg_match('/^Subject: (.*)/i', $line, $matches))
     $Subject = $matches[1];

import re;
    
R = re.compile("^Subject: (.*)", re.IGNORECASE);
M = R.search(line)
if M:
    subject = M.group(1)

$text =~ s{
   \b
   # Capture the address to $1 . . . 
   (
     \w[-.\w]*                          # username
     @
     [-\w]+(\.[-\w]+)*\.(com|edu|info)  # hostname
   )
   \b
}{<a href="mailto:$1">$1</a>}gix;

import java.util.regex.*; // Make regex classes easily available
     
Pattern r = Pattern.compile(
   "\\b                                                  \n"+
   "# Capture the address to $1 . . . \n"+
   "(\n"+
   "  \\w[-.\\w]*                            # username\n"+
   "  @\n"+
   "  [-\\w]+(\\.[-\\w]+)*\\.(com|edu|info)  # hostname\n"+
   ")\n"+
   "\\b\n",
   Pattern.CASE_INSENSITIVE|Pattern.COMMENTS);

Matcher m = r.matcher(text);
text = m.replaceAll("<a href=\"mailto:$1\">$1</a>");

Dim R As Regex = New Regex _
("\b                                                 " & _
 "(?# Capture the address to $1 . . . )                    " & _
 "(                                                                 " & _
 "  \w[-.\w]*                         (?# username)    " & _
 "  @                                                               " & _
 "  [-\w]+(\.[-\w]+)*\.(com|edu|info) (?# hostname)    " & _
 ")                                                                 " & _
 "\b",  _
 RegexOptions.IgnoreCase Or RegexOptions.IgnorePatternWhitespace)

text = R.Replace(text, "<a href=""mailto:${1}"">${1}</a>")

$text = preg_replace('{
                         \b
                         # Capture the address to $1 . . . 
                         (
                           \w[-.\w]*                          # username
                           @
                           [-\w]+(\.[-\w]+)*\.(com|edu|info)  # hostname
                         )
                         \b
                     }ix',
                     '<a href="mailto:$1">$1</a>',  # replacement string
                     $text);

sub(/mizpel/, "misspell")

regsub mizpel $var misspell newvar

regsub -all mizpel $var misspell newvar

(defun FindNextDbl ()
  "move to next doubled word, ignoring <...> tags"   (interactive)
  (re-search-forward "\\<\\([a-z]+\\)\\([\n \t]\\|<[^>]+>\\)+\\1\\>")
)

$str =~ m/(\w+)/;

$regex = '(\w+)';
$str =~ $regex;

$regex = "(\\w+)";
$str =~ $regex;

(?!\p{Cn})\p{InThai}
(?=\P{Cn})\p{InThai}
\p{InThai}(?<!\p{Cn})
\p{InThai}(?<=\P{Cn})

my $need_close_anchor = 0; # True if we've seen <A>, but not its closing </A>.

while (not $html =~ m/\G\z/gc) # While we haven't worked our way to the end . . . 
{
  if ($html =~ m/\G(\w+)/gc) {
     . . . have a word or number in $1 -- can now check for profanity, for example . . . 
  } elsif ($html =~ m/\G[^<>&\w]+/gc) {
    # Other non-HTML stuff -- simply allow it.
  } elsif ($html =~ m/\G<img\s+([^>]+)>/gci) {
     . . . have an image tag -- can check that it's appropriate . . . 
              
  } elsif (not $need_close_anchor and $html =~ m/\G<A\s+([^>]+)>/gci){
     . . . have a link anchor - can validate it . . . 
              
    $need_close_anchor = 1; # Note that we now need </A>
  } elsif ($need_close_anchor and $html =~ m{\G</A>}gci){
    $need_close_anchor = 0; # Got what we needed; don't allow again
  } elsif ($html =~ m/\G&(#\d+|\w+);/gc){
    # Allow entities like &gt; and &#123;
  } else {
    # Nothing matched at this point, so it must be an error. Note the location, and grab a dozen or so
    # characters from the HTML so that we can issue an informative error message.
    my $location = pos($html); # Note where the unexpected HTML starts.
    my ($badstuff) = $html =~ m/\G(.{1,12})/s;
    die "Unexpected HTML at position $location: $badstuff\n";
  }
}

# Make sure there's no dangling <A>
if ($need_close_anchor) {
   die "Missing final </A>"
}

$HostnameRegex = qr/[-a-z0-9]+(\.[-a-z0-9]+)*\.(com|edu|info)/i;

( <A\s+[^>]+> \s* )?  # Match leading <A> tag, if there.
<IMG\s+[^>]+>         # Match <IMG> tag.
(?(1)\s*</A>)         # Match a closing </A>, if we'd matched an <A> before.

echo =XX========================================= | egrep 'X(.+)+X'

The dragging belly indicates your cat is too fat

a 1234 num
a 1234 num
a 1234 num
a 1234 num

The name "McDonald's" is said "makudonarudo" in Japanese

The name "McDonald's" is said "makudonarudo" in Japanese

...<B>Billions</B> and <B>Zillions</B> of suns...

...<B>Billions</B> and <B>Zillions</B> of suns...

...<B>Billions</B> and <B>Zillions</B> of suns...

...<B>Billions and <B>Zillions</B> of suns...

<B>            # Match the opening <B>
(# Now, only as many of the following as needed . . . 
  (?!  <B>  )  #    If not <B> . . . 
  .                          #            . . . any character is okay
)*? #
</B> #  . . . until the closing delimiter can match

<B>               # Match the opening <B>
(# Now, as many of the following as possible . . . 
  (?!  </?B>  )  #    If not <B>, and not </B> . . . 
  .                          #        . . . any character is okay
)* # (now greedy)
</B> # <ANNO> . . . until the closing delimiter can match.

$price =~ s/(\.\d\d[1-9]?)\d*/$1/;

$price =~ s/(\.\d\d[1-9]?)\d+/$1/

SRC=array.c builtin.c eval.c field.c gawkmisc.c io.c main.c \
        missing.c msg.c node.c re.c version.c

SRC=array.c builtin.c eval.c field.c gawkmisc.c io.c main.c \
        missing.c msg.c node.c re.c version.c

$WholePath =~ m{([^/]*)$}; # Check variable $WholePath with regex.
$FileName = $1; # Note text matched

if ( $WholePath =~ m!^(.*)/([^/]*)$! ) {
    # Have a match -- $1 and $2 are valid
    $LeadingPath = $1;
    $FileName = $2;
} else {
    # No match, so there's no `/' in the filename
    $LeadingPath = "."; # so "file.txt" looks like ". / file.txt" ("." is the current directory)
    $FileName = $WholePath;
}

\([^()]*(\([^()]*\)[^()]*)*\)

$regex =  '\(' . '(?:[^()]|\(' x $depth . '[^()]*' . '\))*' x $depth . '\)';

Darth Symbol: "/-|-\\" or "[^-^]"

Darth Symbol: "/-|-\\" or "[^-^]"

"You need a 2\"x3\" photo.

s/^\s+//;
s/\s+$//;

s/^\s+//;
s/\s+$//;

$html =~ s/<[^>]+>//g;

<              # Opening "<"
  (            #    Any amount of . . . 
     "[^"]*"   #      double-quoted string,
     |         #      or . . . 
     '[^']*'   #      single-quoted string,
     |         #      or . . . 
     [^'">]    #      "other stuff"
  )*           #
>              # Closing ">"

...<a href="http://www.oreilly.com">O'Reilly Media</a>...

# Note: the regex in the while(...) is overly simplistic - see text for discussion
while ($Html =~ m{<a\b([^>]+)>(.*?)</a>}ig)
{
  my $Guts = $1; # Save results from the match above, to their own . . . 
  my $Link = $2; #  . . . named variables, for clarity below.

  if ($Guts =~ m{
                 \b HREF       # "href" attribute
                 \s* = \s*     # "=" may have whitespace on either side
                 (?:           # Value is . . . 
                   "([^"]*)"   #   double-quoted string,
                   |           #   or . . . 
                   '([^']*)'   #   single-quoted string,
                   |           #   or . . . 
                   ([^'">\s]+) #   "other stuff"
                 )             #
                }xi)
  {
    my $Url = $+; # Gives the highest-numbered actually-filled $1, $2, etc.
    print "$Url with link text: $Link\n";
  }
}

Imports System.Text.RegularExpressions
   
' Set up the regular expressions we'll use in the loop
Dim A_Regex as Regex = New Regex(                  _
            "<a\b(?<guts>[^>]+)>(?<Link>.*?)</a>", _
            RegexOptions.IgnoreCase)

Dim GutsRegex as Regex = New Regex( _
   "\b HREF                (?#  'href' attribute             )" & _
   "\s* = \s*              (?#  '=' with optional whitespace )" & _
   "(?:                    (?#  Value is ...                 )" & _
   "  ""(?<url>[^""]*)""   (?#    double-quoted string,      )" & _
   "  |                    (?#    or ...                     )" & _
   "  '(?<url>[^']*)'      (?#    single-quoted string,      )" & _
   "  |                    (?#    or ...                     )" & _
   "  (?<url>[^'"">\s]+)   (?#    'other stuff'              )" & _
   ")                      (?#                               )",  _
   RegexOptions.IgnoreCase OR RegexOptions.IgnorePatternWhitespace)

' Now check the 'Html' Variable . . .  
Dim CheckA as Match = A_Regex.Match(Html)

' For each match within . . . 
While CheckA.Success
   ' We matched an <a> tag, so now check for the URL.
   Dim UrlCheck as Match = _
      GutsRegex.Match(CheckA.Groups("guts").Value)
   If UrlCheck.Success
      ' We've got a match, so have a URL/link pair
      Console.WriteLine("Url " & UrlCheck.Groups("url").Value & _
                        " WITH LINK " & CheckA.Groups("Link").Value)
   End If
   CheckA = CheckA.NextMatch
End While

if ($url =~ m{^http://([^/:]+)(:(\d+))?(/.*)?$}i)
{
  my $host = $1;
  my $port = $3 || 80;  # Use $3 if it exists; otherwise default to 80.
  my $path = $4 || "/"; # Use $4 if it exists; otherwise default to "/".
  print "Host: $host\n";
  print "Port: $port\n";
  print "Path: $path\n";
} else {
  print "Not an HTTP URL\n";
}

^
 (?i)  # apply this regex in a case-insensitive manner.
 # One or more dot-separated parts . . . 
 (?: [a-z0-9]\. | [a-z0-9][-a-z0-9]*[a-z0-9]\. )+
 # Followed by the final suffix part . . . 
 (?: com|edu|gov|int|mil|net|org|biz|info|name|museum|coop|aero|[a-z][a-z] )
$

^
 (?i)  # apply this regex in a case-insensitive manner.
 # Zero or more dot-separated parts . . . 
 (?: [a-z0-9]\. | [a-z0-9][-a-z0-9]{0,61}[a-z0-9]\. )*
 # Followed by the final suffix part . . . 
 (?: com|edu|gov|int|mil|net|org|biz|info|name|museum|coop|aero|[a-z][a-z] )
$

...visit us at www.oreilly.com or mail to orders@oreilly.com.

(?i: [a-z0-9] (?:[-a-z0-9]*[a-z0-9])? \. )+ # sub domains
# Now ending .com, etc. For these, we require lowercase
(?-i: com\b
    | edu\b
    | biz\b
    | org\b
    | gov\b
    | in(?:t|fo)\b # .int or .info
    | mil\b
    | net\b
    | name\b
    | museum\b
    | coop\b
    | aero\b
    | [a-z][a-z]\b # two-letter country codes
)

\b
# Match the leading part (proto://hostname, or just hostname)
(
    # ftp://, http://, or https:// leading part
    (ftp|https?)://[-\w]+(\.\w[-\w]*)+
  |
    # or, try to find a hostname with our more specific sub-expression
    full-hostname-regex
)

# Allow an optional port number
( : \d+ )?

# The rest of the URL is optional, and begins with / . . . 
(
   / path-part
)?

Read his comments at http://www.oreilly.com/ask_tim/index.html. He ...

\b
# Match the leading part (proto://hostname, or just hostname)
(
    # ftp://, http://, or https:// leading part
    (ftp|https?)://[-\w]+(\.\w[-\w]*)+
  |
    # or, try to find a hostname with our more specific sub-expression
    (?i: [a-z0-9] (?:[-a-z0-9]*[a-z0-9])? \. )+ # sub domains
    # Now ending .com, etc. For these, require lowercase
    (?-i: com\b
        | edu\b
        | biz\b
        | gov\b
        | in(?:t|fo)\b # .int or .info
        | mil\b
        | net\b
        | org\b
        | [a-z][a-z]\b # two-letter country codes
    )
)

# Allow an optional port number
( : \d+ )?

# The rest of the URL is optional, and begins with / . . . 
(
     /
     # The rest are heuristics for what seems to work well
     [^.!,?;"'<>()\[\]{}\s\x7F-\xFF]*
     (?:
        [.!,?]+  [^.!,?;"'<>()\[\]{}\s\x7F-\xFF]+
     )*
)?

String SubDomain  = "(?i:[a-z0-9]|[a-z0-9][-a-z0-9]*[a-z0-9])";
String TopDomains = "(?x-i:com\\b        \n" +
                    "     |edu\\b        \n" +
                    "     |biz\\b        \n" +
                    "     |in(?:t|fo)\\b \n" +
                    "     |mil\\b        \n" +
                    "     |net\\b        \n" +
                    "     |org\\b        \n" +
                    "     |[a-z][a-z]\\b \n" + // country codes
                    ")                   \n";
String Hostname = "(?:" + SubDomain + "\\.)+" + TopDomains;

String NOT_IN   = ";\"'<>()\\[\\]{}\\s\\x7F-\\xFF";
String NOT_END  = "!.,?";
String ANYWHERE = "[^" + NOT_IN + NOT_END + "]";
String EMBEDDED = "[" + NOT_END + "]";
String UrlPath  = "/"+ANYWHERE + "*("+EMBEDDED+"+"+ANYWHERE+"+)*";
String Url = 
  "(?x:                                                \n"+
  "  \\b                                               \n"+
  "  ## match the hostname part                        \n"+
  "  (                                                 \n"+
  "    (?: ftp | http s? ): // [-\\w]+(\\.\\w[-\\w]*)+ \n"+
  "   |                                                \n"+
  "    " + Hostname + "                                \n"+
  "  )                                                 \n"+
  "  # allow optional port                             \n"+
  "  (?: :\\d+ )?                                      \n"+
  "                                                    \n"+
  "  # rest of url is optional, and begins with /      \n"+
  " (?: " + UrlPath + ")?                              \n"+
  ")";

// Now convert string we've built up into a real regex object
Pattern UrlRegex = Pattern.compile(Url);
// Now ready to apply to raw text to find urls . . . 

@zips = m/(?:\d\d\d\d\d)*?(44\d\d\d)/g;

@zips = m/\G(?:(?!44)\d\d\d\d\d)*(44\d\d\d)/g;

@zips = ( ); # Ensure the array is empty

while (m/(\d\d\d\d\d)/g) {
   $zip = $1;
   if (substr($zip, 0, 2) eq "44") {
       push @zips, $zip;
   }
}

Ten Thousand,10000, 2710 ,,"10,000","It's ""10 Grand"", baby",10K

# Either some non-quote/non-comma text . . . 
[^",]+
# . . . or . . . 
 |
# . . . a double-quoted field (inside, paired double quotes are allowed)
" # field's opening quote
 (?: [^"] | "" )*
" # field's closing quote

# Either some non-quote/non-comma text . . . 
( [^",]+ )
# . . . or . . . 
 |
# . .. a double-quoted field (inside, paired double quotes are allowed)
" # field's opening quote
 (   (?: [^"] | "" )*   )
" # field's closing quote

while ($line =~ m{
           # Either some non-quote/non-comma text . . . 
           ( [^",]+ )
           # . . . or . . . 
            |
           # . . . a double-quoted field ("" allowed inside)
           " # field's opening quote
            (   (?: [^"] | "" )*   )
           " # field's closing quote
        }gx)
{
   if (defined $1) {
       $field = $1;
   } else {
       $field = $2;
       $field =~ s/""/"/g;
   }
   print "[$field]"; # print the field, for debugging
   Can work with $field now . . . 
}

[Ten Thousand][10000][ 2710 ][10,000][It's "10 Grand", baby][10K]

(?:^|,)
(?:
    # Either some non-quote/non-comma text....
    ( [^",]* )
  # ... or...
  |
    # ... a double-quoted field (inside, paired double quotes are allowed)
    " # field's opening quote
     (  (?: [^"] | "" )*   )
    " # field's closing quote
)

(?:^|,)
(?: # Now, match either a double-quoted field (inside, paired double quotes are allowed) . . . 
        " # (double-quoted field's opening quote)
         (   (?: [^"] | "" )*   )
        " # (double-quoted field's closing quote)
  |
    #  . . . or, some non-quote/non-comma text . . . 
        ( [^",]* )
)

import java.util.regex.*;

        

String regex = // Puts a doublequoted field into group(1), an unquoted field into group(2)
   "\\G(?:^|,)                                  \n"+
   "(?:                                         \n"+
   "   # Either a double-quoted field . . .        \n"+
   "   \"  # field's opening quote              \n"+
   "    (   (?: [^\"]++ | \"\" )*+   )          \n"+
   "   \"  # field's closing quote              \n"+
   " |#  . . . or . . .                               \n"+
   "       # some non-quote/non-comma text . . .  \n"+
   "   ( [^\",]* )                              \n"+
   " )                       \n";
// Create a matcher, using the regex above, with dummy text for the time being.
Matcher mMain = Pattern.compile(regex, Pattern.COMMENTS).matcher("");

// Create a matcher for  「"" , with dummy text for the time being
Matcher mQuote = Pattern.compile("\"\"").matcher("");

        

// Above is the preparation; the code below is executed on a per-line basis
mMain.reset(line); // Use this line of CSV text in the processing below

while (mMain.find())
{
    String field;
    if (mMain.start(2) >= 0)
        field = mMain.group(2); // The field is unquoted, so we can use it as is
    else
        // The field is quoted, so we must replace paired doublequotes with one double quote
        field = mQuote.reset(mMain.group(1)).replaceAll("\"");

    // We can now work with field . . . 
    System.out.println("Field [" + field + "]");
}

Imports System.Text.RegularExpressions
   
Dim FieldRegex as Regex = New Regex( _
       "(?:^|,)                                        " & _
       "(?:                                            " & _
       "   (?# Either a doublequoted field ...)        " & _
       "   ""  (?# field's opening quote )             " & _
       "    (   (?> [^""]+ | """" )*   )               " & _
       "   ""  (?# field's closing quote )             " & _
       " (?# ... or ...)                               " & _
       " |                                             " & _
       "   (?# ... some non-quote/non-comma text ...)  " & _
       "   ( [^"",]* )                                 " & _
       " )", RegexOptions.IgnorePatternWhitespace)

Dim QuotesRegex as Regex = New Regex("""""") 'A string with two double quotes
    
Dim FieldMatch as Match = FieldRegex.Match(Line)
While FieldMatch.Success
   Dim Field as String
   If FieldMatch.Groups(1).Success
     Field = QuotesRegex.Replace(FieldMatch.Groups(1).Value, """")
   Else
     Field = FieldMatch.Groups(2).Value
   End If

   Console.WriteLine("[" & Field & "]")
   ' Can now work with 'Field'....

   FieldMatch = FieldMatch.NextMatch
End While

"You need a 2\"3\" photo."

use Time::HiRes 'time'; # So time() gives a high-resolution value.

$StartTime = time();
"abababdedfg" =~ m/^(a|b|c|d|e|f|g)+$/;
$EndTime = time();
printf("Alternation takes %.3f seconds.\n", $EndTime - $StartTime);

$StartTime = time();
"abababdedfg" =~ m/^[a-g]+$/;
$EndTime = time();
printf("Character class takes %.3f seconds.\n", $EndTime - $StartTime);

use Time::HiRes 'time'; # So time() gives a high-resolution value.
$TimesToDo = 1000;                  # Simple setup
$TestString = "abababdedfg" x 1000; # Makes a huge string

$Count = $TimesToDo;
$StartTime = time();
while ($Count-- > 0) {
      $TestString =~ m/^(a|b|c|d|e|f|g)+$/;
}
$EndTime = time();
printf("Alternation takes %.3f seconds.\n", $EndTime - $StartTime);

$Count = $TimesToDo;
$StartTime = time();
while ($Count-- > 0) {
      $TestString =~ m/^[a-g]+$/;
}
$EndTime = time();
printf("Character class takes %.3f seconds.\n", $EndTime - $StartTime);

$TimesToDo = 1000000;
$TestString = "abababdedfg";

$TimesToDo = 1000;

/* Prepare the test string */
$TestString = "";
for ($i = 0; $i < 1000; $i++)
    $TestString .= "abababdedfg";

/* Do the first test */
$start = gettimeofday();
for ($i = 0; $i < $TimesToDo; $i++)
     preg_match('/^(a|b|c|d|e|f|g)+$/', $TestString);
$final = gettimeofday();
$sec = ($final['sec'] + $final['usec']/1000000) -
       ($start['sec'] + $start['usec']/1000000);
printf("Alternation takes %.3f seconds\n", $sec);

/* And now the second test */
$start = gettimeofday();
for ($i = 0; $i < $TimesToDo; $i++)
     preg_match('/^[a-g]+$/', $TestString);
$final = gettimeofday();
$sec = ($final['sec'] + $final['usec']/1000000) -
       ($start['sec'] + $start['usec']/1000000);
printf("Character class takes %.3f seconds\n", $sec);

if (phpversion() >= 5)
   date_default_timezone_set("GMT");

import java.util.regex.*;
public class JavaBenchmark {
 public static void main(String [] args)
 {
   Matcher regex1 = Pattern.compile("^(a|b|c|d|e|f|g)+$").matcher("");
   Matcher regex2 = Pattern.compile("^[a-g]+$").matcher("");
   long timesToDo = 1000;

   StringBuffer temp = new StringBuffer();
   for (int i = 1000; i > 0; i--)
           temp.append("abababdedfg");
   String testString = temp.toString();

   // Time first one . . . 
   long count = timesToDo;
   long startTime = System.currentTimeMillis();
   while (--count > 0)
         regex1.reset(testString).find();
   double seconds = (System.currentTimeMillis() - startTime)/1000.0;
   System.out.println("Alternation takes " + seconds + " seconds");

   // Time second one . . . 
   count = timesToDo;
   startTime = System.currentTimeMillis();
   while (--count > 0)
         regex2.reset(testString).find();
   seconds = (System.currentTimeMillis() - startTime)/1000.0;
   System.out.println("Character class takes " + seconds + " seconds");
 }
}

// Time first one . . . 
for (int i = 4; i > 0; i--)
{
    long count = timesToDo;
    long startTime = System.currentTimeMillis();
    while (--count > 0)
          regex1.reset(testString).find();
    double seconds = (System.currentTimeMillis() - startTime)/1000.0;
    System.out.println("Alternation takes " + seconds + " seconds");
}

Option Explicit On
Option Strict On

Imports System.Text.RegularExpressions

Module Benchmark
Sub Main()
  Dim Regex1 as Regex = New Regex("^(a|b|c|d|e|f|g)+$")
  Dim Regex2 as Regex = New Regex("^[a-g]+$")
  Dim TimesToDo as Integer = 1000
  Dim TestString as String = ""
  Dim I as Integer
  For I = 1 to 1000
     TestString = TestString & "abababdedfg"
  Next

  Dim StartTime as Double = Timer()
  For I = 1 to TimesToDo
     Regex1.Match(TestString)
  Next
  Dim Seconds as Double = Math.Round(Timer() - StartTime, 3)
  Console.WriteLine("Alternation takes " & Seconds & " seconds")

  StartTime = Timer()
  For I = 1 to TimesToDo
     Regex2.Match(TestString)
  Next
  Seconds = Math.Round(Timer() - StartTime, 3)
  Console.WriteLine("Character class takes " & Seconds & " seconds")
End Sub
End Module

TimesToDo=1000
testString=""
for i in 1..1000
    testString += "abababdedfg"
end

Regex1 = Regexp::new("^(a|b|c|d|e|f|g)+$");
Regex2 = Regexp::new("^[a-g]+$");

startTime = Time.new.to_f
for i in 1..TimesToDo
    Regex1.match(testString)
end
print "Alternation takes %.3f seconds\n" % (Time.new.to_f - startTime);

startTime = Time.new.to_f
for i in 1..TimesToDo
    Regex2.match(testString)
end
print "Character class takes %.3f seconds\n" % (Time.new.to_f - startTime);

import re
import time
import fpformat

Regex1 = re.compile("^(a|b|c|d|e|f|g)+$")
Regex2 = re.compile("^[a-g]+$")

TimesToDo = 1250;
TestString = ""
for i in range(800):
    TestString += "abababdedfg"

StartTime = time.time()
for i in range(TimesToDo):
   Regex1.search(TestString)
Seconds = time.time() - StartTime
print "Alternation takes " + fpformat.fix(Seconds,3) + " seconds"

StartTime = time.time()
for i in range(TimesToDo):
   Regex2.search(TestString)
Seconds = time.time() - StartTime
print "Character class takes " + fpformat.fix(Seconds,3) + " seconds"

set TimesToDo 1000
set TestString ""
for {set i 1000} {$i > 0} {incr i -1} {
    append TestString "abababdedfg"
}

set Count $TimesToDo
set StartTime [clock clicks -milliseconds]
for {} {$Count > 0} {incr Count -1} {
    regexp {^(a|b|c|d|e|f|g)+$} $TestString
}
set EndTime [clock clicks -milliseconds]
set Seconds [expr ($EndTime - $StartTime)/1000.0]
puts [format "Alternation takes %.3f seconds" $Seconds]

set Count $TimesToDo
set StartTime [clock clicks -milliseconds]
for {} {$Count > 0} {incr Count -1} {
    regexp {^[a-g]+$} $TestString
}
set EndTime [clock clicks -milliseconds]
set Seconds [expr ($EndTime - $StartTime)/1000.0]
puts [format "Character class takes %.3f seconds" $Seconds]

while (...) {
    if ($line =~ m/^\s*$/ ) ...
    if ($line =~ m/^Subject: (.*)/) ...
    if ($line =~ m/^Date: (.*)/) ...
    if ($line =~ m/^Reply-To: (\S+)/)...
    if ($line =~ m/^From: (\S+) \(([^()]*)\)/)...
      
}

if ($data =~ m/\(0x/
    and
    $data =~ m/(?:SCALAR|ARRAY|...|HASH)\(0x[0-9a-fA-F]+\)/)
{
   # warn about bogus data...
}

<B>               # Match the opening <B>
(# Now, as many of the following as possible . . . 
  (?!  </?B>  )  #    If not <B>, and not </B> . . . 
  .                          #        . . . any character is okay
)* # (now greedy)
</B> # <ANNO> . . . until the closing delimiter can match.

<B>              # Match the opening <B>
  (?> [^<]* )    # Now match any "normal" . . . 
  (?>                   # Any amount of . . . 
     (?! </?B> ) #   if not at <B> or </B>,
     <           #   match one "special"
     [^<]*       #   and then any amount of "normal"
   )*                   #
</B>             # And finally the closing </B>

^ \w+ =                 # leading field name and '='
# Now read (and capture) the value . . . 
(
   (?> [^\n\\]* )       # "normal"*
   (?> \\. [^\n\\]* )*  # ( "special" "normal"* )*
)

(?:^|,)
(?: # Now, match either a double-quoted field (inside, paired double quotes are allowed) . . . 
        " # (double-quoted field's opening quote)
         (   (?: [^"] | "" )*   )
        " # (double-quoted field's closing quote)
  |
    #  . . . or, some non-quote/non-comma text . . . 
        ( [^",]* )
)

while ($line =~ m{
          \G(?:^|,)
          (?:
             # Either a double-quoted field (with "" for each ")...
             " # field's opening quote
              ( (?> [^"]* ) (?> "" [^"]* )*  )
             " # field's closing quote
           # ..or...
           |
             # ... some non-quote/non-comma text....
             ( [^",]* )
          )
      }gx)
{
   if (defined $2) {
       $field = $2;
   } else {
       $field = $1;
       $field =~ s/""/"/g;
   }
   print "[$field]"; # print the field, for debugging
   Can work with $field now . . . 
}

years = days /x divide x//365; /x assume non-leap year x/

const char *cstart = "/*", *cend = "*/";

$prog =~ s{/\*[^*]*\*+(?:[^/*][^*]*\*+)*/}{}g; # remove C comments (and more!)

char *CommentStart = "/*";  /* start of comment */
char *CommentEnd   = "*/";  /* end of comment */

$COMMENT = qr{/\*[^*]*\*+(?:[^/*][^*]*\*+)*/}; # regex to match a comment
$DOUBLE = qr{"(?:\\.|[^\\"])*"};# regex to match double-quoted string
$text =~ s/$DOUBLE|$COMMENT//g;

$COMMENT = qr{/\*[^*]*\*+(?:[^/*][^*]*\*+)*/}; # regex to match a comment
$DOUBLE = qr{"(?:\\.|[^\\"])*"};# Regex to match double-quoted string
$text =~ s/($DOUBLE)|$COMMENT/$1/g;

$text =~ s/($DOUBLE)|$COMMENT/defined($1) ? $1 : ""/ge;

$COMMENT = qr{/\*[^*]*\*+(?:[^/*][^*]*\*+)*/}; # regex to match a comment
$COMMENT2 = qr{//[^\n]*};  # regex to match a C++ // comment
$DOUBLE = qr{"(?:\\.|[^\\"])*"};# regex to match double-quoted string
$SINGLE = qr{'(?:\\.|[^'\\])*'};# regex to match single-quoted string

$text =~ s/($DOUBLE|$SINGLE)|$COMMENT|$COMMENT2/$1/g;

$OTHER = qr{[^"'/]};  # Stuff that couldn't possibly begin one of the other alternatives
  
$text =~ s/($DOUBLE|$SINGLE|$OTHER+)|$COMMENT|$COMMENT2/$1/g;

$DOUBLE = qr{"[^\\"]*(?:\\.[^\\"]*)*"};
$SINGLE = qr{'[^'\\]*(?:\\.[^'\\]*)*'};

([^"'/]+|"[^\\"]*(?:\\.[^\\"]*)*"[^"'/]*|'[^'\\]*
(?:\\.[^'\\]*)*'[^"'/]*)|/\*[^*]*\*+(?:[^/*][^*]*\*+)*/|//[^\n]*

$MatchField = "^Subject:"; # Normal string assignment
   
if ($text =~ $MatchField) {

$text =~ $MatchField

$text =~ m/$MatchField/

use Config;
print "$Config{privlib}/unicore/UnicodeData.txt\n";

m{
    regex  # comments
    here   # here
}x;

$text =~ m/.../;
$text =~  /.../;

$s = expression one;
@a = expression two;

$var = ($this, &is, 0xA, 'list');

{
    local($Acme::Widget::Debug) = 1; # Ensure it's turned on
    # work with Acme::Widget while debugging is on
      
}
# $Acme::Widget::Debug is now back to whatever it had been before

{
    local $^W = 0; # Ensure warnings are off.
    UnrulyFunction(...);
}
# Exiting the block restores the original value of $^W.

if (m/(...)/)
{
    DoSomeOtherStuff();
    print "the matched text was $1.\n";
}

if ($result =~ m/ERROR=(.*)/) {
   warn "Hey, tell $Config{perladmin} about $1!\n";
}

"Pi is 3.14159, roughly" =~ m/\b(;(;tasty|fattening);|(;\d+(;\.\d*);?););\b/;

$url =~ m{
   href \s* = \s*   # Match the "href = " part, then the value . . . 
   (?: "([^"]*)"    # a double-quoted value, or . . . 
     | '([^']*)'    # a single-quoted value, or . . . 
     | ([^'"<>]+) ) # an unquoted value.
}ix;

$text = "Version 6 coming soon?";
   
$text =~ m/\d+/;

1 while $line =~ s/\t/' ' x (8 - $-[0] % 8)/e;

my $HostnameRegex = qr/[-a-z0-9]+(?:\.[-a-z0-9]+)*\.(?:com|edu|info)/i;

my $HttpUrl = qr{
   http:// $HostnameRegex \b  # Hostname
   (?:
        / [-a-z0-9_:\@&?=+,.!/~*'%\$]* # Optional path
           (?<![.,?!])                 # Not allowed to end with [.,?!]
   )?
}ix;

if ($text =~ $HttpUrl) {
   print "There is a URL\n";
}

while ($text =~ m/($HttpUrl)/g) {
   print "Found URL: $1\n";
}

my $HostnameRegex = qr{
   # One or more dot-separated parts...
   (?: [a-z0-9]\. | [a-z0-9][-a-z0-9]{0,61}[a-z0-9]\. )*
   # Followed by the final suffix part...
   (?: com|edu|gov|int|mil|net|org|biz|info|...|aero|[a-z][a-z] )
}xi;

my $WordRegex = qr/\b \w+ \b/; # Oops, missing the /x modifier!
   
if ($text =~ m/^($WordRegex)/x) {
    print "found word at start of text: $1\n";
}

my $WordRegex = qr/\b \w+ \b/x;  # This works!
   
if ($text =~ m/^($WordRegex)/) {
    print "found word at start of text: $1\n";
}

my $WordRegex = '\b \w+ \b';  # Normal string assignment
   
if ($text =~ m/^($WordRegex)/x) {
    print "found word at start of text: $1\n";
}

my $WordRegex = '(?x:\b \w+ \b)'; # Normal string assignment
   
if ($text =~ m/^($WordRegex)/) {
    print "found word at start of text: $1\n";
}

(?ix-sm:
   http:// (?ix-sm:
   # One or more dot-separated parts...
   (?: [a-z0-9]\. | [a-z0-9][-a-z0-9]{0,61}[a-z0-9]\. )*
   # Followed by the final suffix part...
   (?: com|edu|gov|int|mil|net|org|biz|info|...|aero|[a-z][a-z] )
) \b          # hostname
   (?:
        / [-a-z0-9_:\@&?=+,.!/~*'%\$]* # Optional path
           (?<![.,?!])                 # Not allowed to end with [.,?!]
   )?
)

$text =~ m/regex/

StringOperand =~ RegexOperand

my $regex = qr/regex/;
  
if ($text =~ $regex) {

if ($text =~ m/$regex/) {

$text =~ m/.../;   # Just do it, presumably, for the side effects.
 
if ($text =~ m/.../) {
  # Do code if match is successful
 
 
$result = ( $text =~ m/.../ ); # Set $result to result of match against $text
$result =   $text =~ m/.../  ; # Same thing; =~ has higher precedence than = 
 
  $copy = $text;             # Copy $text to $copy ...
  $copy           =~ m/.../;# ... and perform match on $copy
( $copy = $text ) =~ m/.../;# Same thing in one expression

$text =~ m/regex/;

$text = m/regex/;

$text =        m/regex/;
$text = ($_ =~ m/regex/);

while (<>)
{
   if (m/.../) {
     
   } elsif (m/.../) {

if ($text !~ m/.../)

if (not $text =~ m/.../)

unless ($text =~ m/.../)

if ($target =~ m/.../) {
    #  . . . processing after successful match . . . 
 
} else {
    #  . . . processing after unsuccessful match . . . 
 
}

my $success  =  $target =~ m/.../;
  
if ($success) {
  
}

my ($year, $month, $day)  =  $date =~ m{^ (\d+) / (\d+) / (\d+) $}x;

my @parts  =  $text =~ m/^(\d+)-(\d+)-(\d+)$/;

my ($word)   =  $text =~ m/(\w+)/;
my $success  =  $text =~ m/(\w+)/;

if ( my ($year, $month, $day) = $date =~ m{^ (\d+) / (\d+) / (\d+) $}x ) {
    # Process for when we have a match: $year and such are available
} else {
    # here if no match . . . 
}

my @nums  =  $text =~ m/\d+/g;

my $hex_ip = join '', map { sprintf("%02x", $_) } $ip =~ m/\d+/g;

my $ip = join '.', map { hex($_) } $hex_ip =~ m/../g

my @nums  =  $text =~ m/\d+(?:\.\d+)?|\.\d+/g;

my @Tags  =  $Html =~ m/<(\w+)/g;

alias  Jeff      jfriedl@regex.info
alias  Perlbug   perl5-porters@perl.org
alias  Prez      president@whitehouse.gov

( 'Jeff', 'jfriedl@regex.info', 'Perlbug',
  'perl5-porters@perl.org', 'Prez', 'president@whitehouse.gov' )

my %alias  =  $text =~ m/^alias\s+(\S+)\s+(.+)/mg;

$text = "WOW! This is a SILLY test.";

$text =~ m/\b([a-z]+\b)/g;
print "The first all-lowercase word: $1\n";

$text =~ m/\b([A-Z]+\b)/g;
print "The subsequent all-uppercase word: $1\n";

while ($ConfigData =~ m/^(\w+)=(.*)/mg) {
    my($key, $value) = ($1, $2);
      
}

while ($text =~ m/(\d+)/) { # dangerous!
    print "found: $1\n";
}

while ($text =~ m/(\d+)/g) {
    print "found: $1\n";
}

my $ip = "64.156.215.240";
while ($ip =~ m/(\d+)/g) {
   printf "found '$1' ending at location %d\n", pos($ip);
}

if ($logline =~ m/^.{32}(\S+)/) {
    $RequestedPage = $1;
}

pos($logline) = 32; # The page starts at the 32nd character, so start the next match there . . . 
if ($logline =~ m/(\S+)/g) {
    $RequestedPage = $1;
}

pos($logline) = 32; # The page starts at the 32nd character, so start the next match there . . . 
if ($logline =~ m/\G(\S+)/g) {
    $RequestedPage = $1;
}

while (not $html =~ m/\G\z/gc) # While we haven't worked to the end . . . 
{
  if    ($html =~ m/\G( <[^>]+>   )/xgc) { print "TAG: $1\n"            }
  elsif ($html =~ m/\G( &\w+;     )/xgc) { print "NAMED ENTITY: $1\n"   }
  elsif ($html =~ m/\G( &\#\d+;   )/xgc) { print "NUMERIC ENTITY: $1\n" }
  elsif ($html =~ m/\G( [^<>&\n]+ )/xgc) { print "TEXT: $1\n"           }
  elsif ($html =~ m/\G  \n         /xgc) { print "NEWLINE\n"            }
  elsif ($html =~ m/\G( .         )/xgc) { print "ILLEGAL CHAR: $1\n"   }
  else {
      die "$0: oops, this shouldn't happen!";
  }
}

$html =~ m/\G ( <script[^>]*>.*?</script> )/xgcsi

while ("Larry Curly Moe" =~ m/\w+/g) {
   print "WHILE stooge is $&.\n";
}
print "\n";

if ("Larry Curly Moe" =~ m/\w+/g) {
   print "IF stooge is $&.\n";
}
print "\n";

foreach ("Larry Curly Moe" =~ m/\w+/g) {
   print "FOREACH stooge is $&.\n";
}

$text =~ s/regex/replacement/modifiers

$text =~ s{
  ...some big regex here, with lots of comments and such...
} {
  ...a Perl code snippet to be evaluated to produce the replacement text...
}ex;

$text =~ s/-time-/localtime/ge;

$url =~ s/([^a-zA-Z0-9])/sprintf('%%%02x', ord($1))/ge;

$url =~ s/%([0-9a-f][0-9a-f])/pack("C", hex($1))/ige;

$data =~ s/(\$[a-zA-Z_]\w*)/$1/eeg;

@Paragraphs = split(m/\s*<p>\s*/i, $html);

@Lines = split(m/^/m, $lines);

split(match operand, target string, chunk-limit operand)

($var1, $var2, $var3, ...) = split(...);
 
@array = split(...);
 
for my $item (split(...)) {
   
}

( 'IO.SYS', '225558', '95-10-03:-a-sh:optional' )

('IO.SYS', '225558', '95-10-03', '-a-sh:optional')

($filename, $size, $date) = split(/:/, $text);

@nums = split(m/:/, "12:34::78");

("12", "34", "", "78")

@nums = split(m/:/, "12:34::78:::");

("12", "34", "", "78")

my @NonEmpty = grep { length } split(/:/, $text);

@nums = split(m/:/, ":12:34::78");

("", "12", "34", "", "78")

... and <B>very <FONT color=red>very</FONT> much</B> effort...

( '... and ', '<B>', 'very ', '<FONT color=red>',
  'very', '</FONT>', ' much', '</B>', ' effort...' )

( '... and ', 'very ', 'very', ' much', ' effort...' )

"have a nice day" =~ m{
   (?{ print "Starting match.\n" })
   \b(?: the | an | a )\b
}x;

my $Level0 = qr/ \(  ( [^()] )*  \) /x; # Parenthesized text
  
if ($text =~ m/\b( \w+$Level0 )/x) {
   print "found function call: $1\n";
}

my $Level0 = qr/ \(  ( [^()]          )*  \) /x; # Parenthesized text
my $Level1 = qr/ \(  ( [^()]| $Level0 )*  \) /x; # One level of nesting

my $Level0 = qr/ \(  ( [^()]           )*  \) /x; # Parenthesized text
my $Level1 = qr/ \(  ( [^()] | $Level0 )*  \) /x; # One level of nesting
my $Level2 = qr/ \(  ( [^()] | $Level1 )*  \) /x; # Two levels of nesting

my $Level3 = qr/ \(  ( [^()] | $Level2 )*  \) /x; # Three levels of nesting
my $Level4 = qr/ \(  ( [^()] | $Level3 )*  \) /x; # Four levels of nesting
my $Level5 = qr/ \(  ( [^()] | $Level4 )*  \) /x; # Five levels of nesting

my $LevelN; # This must be predeclared because it's used in its own definition.
$LevelN = qr/ \(( [^()] | (??{ $LevelN }) )* \) /x;

if ($text =~ m/\b( \w+$LevelN )/x) {
   print "found function call: $1\n";
}

$LevelN = qr/ (?> [^()]+ | \( (??{ $LevelN }) \)  )* /x;

if ($text =~ m/\b( \w+ \( $LevelN \) )/x) {
   print "found function call: $1\n";
}
 
if (not $text =~ m/^ $LevelN $/x) {
   print "mismatched parentheses!\n";
}

"abcdefgh" =~ m{
  (?{ print "starting match at [$`|$']\n" })
  (?:d|e|f)
}x;

print "starting match at [$`|$']\n"

(?{ print "matched at [$`<$&>$']\n" })

"abcdefgh" =~ m{
  (?{  print "starting match at [$`|$']\n" })
  [def]
}x;

panic: top_env

"oneselfsufficient" =~ m{
    one(self)?(selfsufficient)?
   (?{ print "matched at [$`<$&>$']\n" })
}x;

"123" =~ m{
   \d+
   (?{ print "matched at [$`<$&>$']\n" })
   (?!)
}x;

$longest_match = undef; # We'll keep track of the longest match here

"oneselfsufficient" =~ m{
   one(self)?(selfsufficient)?
   (?{
      # Check to see if the current match ($&) is the longest so far
      if (not defined($longest_match)
          or
          length($&) > length($longest_match))
      {
          $longest_match = $&;
      }
   })
   (?!) # Force failure so we'll backtrack to find further "matches"
}x;

# Now report the accumulated result, if any
if (defined($longest_match)) {
   print "longest match=[$longest_match]\n";
} else {
   print "no match\n";
}

my $RecordPossibleMatch = qr{
   (?{
      # Check to see if the current match ($&) is the longest so far
      if (not defined($longest_match)
          or
          length($&) > length($longest_match))
      {
          $longest_match = $&;
      }
   })
   (?!) # Force failure so we'll backtrack to find further "matches"
}x;

$longest_match = undef; # We'll keep track of the longest match here

"800-998-9938" =~ m{  \d+  $RecordPossibleMatch  }x;

# Now report the accumulated result, if any
if (defined($longest_match)) {
   print "longest match=[$longest_match]\n";
} else {
   print "no match\n";
}

my $BailIfAnyMatch = qr/(?(?{ defined $longest_match})(?!))/;

"800-998-9938" =~ m{ $BailIfAnyMatch  \d+  $RecordPossibleMatch  }x;

my $Count = 0;

$text =~ m{
   ^ (?> \d+ (?{ $Count++ }) \b | \w+ | \s+ )* $
}x;

our $Count = 0;

$text =~ m{
 ^ (?> \d+ (?{ local($Count) = $Count + 1 }) \b | \w+ | \s+ )* $
}x;

m{   (?{ print "starting\n" })  some regex...  }x;

my $ShowStart = '(?{ print "starting\n" })';
    
m{ $ShowStart some regex...  }x;

use re 'eval';

my $Count = undef;
our $TmpCount = 0;

$text =~ m{
 ^ (?> \d+ (?{ local($TmpCount) = $TmpCount + 1 }) \b | \w+ | \s+ )* $
 (?{ $Count = $TmpCount }) # Save the "ending" $Count to a non-localized variable
}x;
if (defined $Count) {
    print "Count is $Count.\n";
} else {
    print "no match\n";
}

sub CheckOptimizer
{
    my $text  = shift; # The first argument is the text to check.
    my $start = undef; # We'll note here where the regex is first applied.

    my $match = $text =~ m{
      (?{ $start = $-[0] if not defined $start}) # Save the first starting position
      \d # This is the regex being tested
    }x;

    if (not defined $start) {
        print "The whole match was optimized away.\n";
        if ($match) {
            # This can't possibly happen!
            print "Whoa, but it matched! How can this happen!?\n";
        }
    } elsif ($start == 0) {
        print "The match start was not optimized.\n";
    } else {
        print "The optimizer started the match at character $start.\n"
    }
}

CheckOptimizer("test 123");

The optimizer started the match at character 5.

The whole match was optimized away.
Whoa, but it matched! How can this happen!?

my $NestedGuts = qr{
  (?>
    (?:
       # Stuff not parenthesis
        [^()]+
       # An opening parenthesis
       |  \(
       # A closing parenthesis
       |  \)
    )*
  )
}x;

(?{ local $OpenParens = 0 })

(?{ $OpenParens++ })

(?(?{ $OpenParens }) (?{ $OpenParens-- }) | (?!) )

(?(?{ $OpenParens != 0 })(?!))

my $NestedGuts = qr{
  (?{ local $OpenParens = 0 }) #  Counts the number of nested opens waiting to close.
  (?> # atomic-grouping for efficiency
     (?:
        # Stuff not parenthesis
          [^()]+
        #  An opening parenthesis
        |  \(   (?{ $OpenParens++ })
        #  Allow a closing parenthesis, if we're expecting any
        |  \)  (?(?{ $OpenParens != 0 }) (?{ $OpenParens-- }) | (?!) )
     )*
  )
  (?(?{ $OpenParens != 0 })(?!)) #  If there are any open parens left, don't finish
}x;

sub MungeRegexLiteral($)
{
   my ($RegexLiteral) = @_; # Argument is a string
   $RegexLiteral =~ s/\\</(?<!\\w)(?=\\w)/g; # Mimic \< as start-of-word boundary
   $RegexLiteral =~ s/\\>/(?<=\\w)(?!\\w)/g; # Mimic \> as end-of-word boundary
   return $RegexLiteral; # Return possibly-modified string
}

package MyRegexStuff; # Best to call the package something unique
use strict;   # Good practice to always use this
use warnings; # Good practice to always use this
use overload; # Allows us to invoke Perl's overloading mechanism
# Have our regex handler installed when we're use'd . . . .
sub import { overload::constant qr => \&MungeRegexLiteral }

sub MungeRegexLiteral($)
{
   my ($RegexLiteral) = @_; # Argument is a string
   $RegexLiteral =~ s/\\</(?<!\\w)(?=\\w)/g; # Mimic \< as start-of-word boundary
   $RegexLiteral =~ s/\\>/(?<=\\w)(?!\\w)/g; # Mimic \> as end-of-word boundary
   return $RegexLiteral; # Return possibly-modified string
}

1; # Standard idiom so that a 'use' of this file returns something true

use lib '.';      # Look for library files in the current directory
use MyRegexStuff; # We now have our new functionality available!
   
$text =~ s/\s+\</ /g; # Normalize any type of whitespace before a word to a single space

$RegexLiteral =~ s/(  \( $LevelN \)[*+?]  )\+/(?>$1)/gx;

$text =~ s/"(\\.|[^"])*+"//; # Remove double-quoted strings

$RegexLiteral =~ s{
 (
   # Match something that can be quantified . . . 
   (?:  \\[\\abCdDefnrsStwWX] # \n, \w, etc.
     |  \\c.                  # \cA
     |  \\x[\da-fA-F]{1,2}    # \xFF
     |  \\x\{[\da-fA-F]*\}    # \x{1234}
     |  \\[pP]\{[^{}]+\}      # \p{Letter}
     |  \[\]?[^]]+\]          # "poor man's" class
     |  \\\W                  # \*
     |  \( $LevelN \)         # (...)
     |  [^()*+?\\]            # almost anything else
    )
    #  . . . and is quantified . . . 
    (?: [*+?] | \{\d+(?:,\d*)?\} )
 )
 \+  #  . . . and has an extra '+' after the quantifier.
}{(?>$1)}gx;

my $SaveUrl = qr{
    ($HttpUrl)         # Match an HTTP URL . . . 
    (?{ $url = $^N })  #  . . . and save to $url
}x;

$text =~ m{
   http \s*=\s* ($SaveUrl)
 | src  \s*=\s* ($SaveUrl)
}xi;

package MyRegexStuff;
use strict;
use warnings;
use overload;
sub import { overload::constant('qr' => \&MungeRegexLiteral) }

my $NestedStuffRegex; # This should be predeclared, because it's used in its own definition.
$NestedStuffRegex = qr{
 (?>
   (?:  # Stuff not parens, not '#', and not an escape . . . 
        [^()\#\\]+
        # Escaped stuff . . . 
      | (?s: \\. )
        # Regex comment . . . 
      | \#.*\n
        # Matching parens, with more nested stuff inside . . . 
      |  \(  (??{ $NestedStuffRegex })   \)
   )*
 )
}x;

sub SimpleConvert($); # This must be predeclared, as it's used recursively
sub SimpleConvert($)
{
  my $re = shift;  # Regex to mangle
  $re =~ s{
      \(\?                    #  "(?"
        <  ( (?>\w+) ) >      #     < $1 > $1 is an identifier
        ( $NestedStuffRegex ) #     $2 - possibly-nested stuff
      \)                      #  ")"
  }{
    my $id   = $1;
    my $guts = SimpleConvert($2);
    # We change
    #    (?<id>guts)
    # to
    #    (?: (guts)  # match the guts
    #        (?{
    #           local($^N{$id}) = $guts # Save to a localized element of %^T
    #         })
    #     )
    "(?:($guts)(?{ local(\$^T{'$id'}) = \$^N }))"
  }xeog;
  return $re;  # Return mangled regex
}

sub MungeRegexLiteral($)
{
  my ($RegexLiteral) = @_; # Argument is a string
  # print "BEFORE: $RegexLiteral\n"; # Uncomment this for debugging
  my $new = SimpleConvert($RegexLiteral);
  if ($new ne $RegexLiteral)
  {
     my $before = q/(?{ local(%^T) = () })/; # Localize temporary hash
     my $after  = q/(?{ %^N = %^T       })/; # Copy temp to "real" hash
     $RegexLiteral = "$before(?:$new)$after";
  }
  # print "AFTER:  $RegexLiteral\n"; # Uncomment this for debugging
  return $RegexLiteral;
}

1;

$ip = sprintf("%03d.%03d.%03d.%03d", split(/\./, $ip));

$ip = sprintf("%03d.%03d.%03d.%03d", split(m/\./, $ip));

substr($ip,  0, 0) = '0' if substr($ip,  1, 1) eq '.';
substr($ip,  0, 0) = '0' if substr($ip,  2, 1) eq '.';
substr($ip,  4, 0) = '0' if substr($ip,  5, 1) eq '.';
substr($ip,  4, 0) = '0' if substr($ip,  6, 1) eq '.';
substr($ip,  8, 0) = '0' if substr($ip,  9, 1) eq '.';
substr($ip,  8, 0) = '0' if substr($ip, 10, 1) eq '.';
substr($ip, 12, 0) = '0' while length($ip) < 15;

$ip = sprintf("%03d.%03d.%03d.%03d", $ip =~ m/\d+/g);

$ip = sprintf("%03d.%03d.%03d.%03d", $ip =~ m/(\d+)/g);

$ip = sprintf("%03d.%03d.%03d.%03d",
              $ip =~ m/^(\d+)\.(\d+)\.(\d+)\.(\d+)$/);

$ip =~ s/\b(?=\d\b)/00/g;
$ip =~ s/\b(?=\d\d\b)/0/g;

$ip =~ s/\b(\d(\d?)\b)/$2 eq '' ? "00$1" : "0$1"/eg;

$ip =~ s/\d+/sprintf("%03d", $&)/eg;

$ip =~ s/(?:(?<=\.)|^)(?=\d\b)/00/g;
$ip =~ s/(?:(?<=\.)|^)(?=\d\d\b)/0/g;

$ip =~ s/\b(\d\d?\b)/'0' x (3-length($1)) . $1/eg;

$ip =~ s/\b(\d\b)/00$1/g;
$ip =~ s/\b(\d\d\b)/0$1/g;

$ip =~ s/\b(\d\d?\b)/sprintf("%03d", $1)/eg;

$ip =~ s/\b(\d{1,2}\b)/sprintf("%03d", $1)/eg;

$ip =~ s/(\d+)/sprintf("%03d", $1)/eg;

$ip =~ s/\b(\d\d?(?!\d))/sprintf("%03d", $1)/eg;

$ip =~ s/(?:(?<=\.)|^)(\d\d?(?!\d))/sprintf("%03d", $1)/eg;

my $today = (qw<Sun Mon Tue Wed Thu Fri Sat>)[(localtime)[6]];
# $today now holds the day ("Mon", "Tue", etc., as appropriate)

while (<LOGFILE>) {
    if (m/^$today:/i) {

my $today = (qw<Sun Mon Tue Wed Thu Fri Sat>)[(localtime)[6]];

while (<LOGFILE>) {
    if (m/^$today:/io) {

sub CheckLogfileForToday()
{
  my $today = (qw<Sun Mon Tue Wed Thu Fri Sat>)[(localtime)[6]];

  while (<LOGFILE>) {
      if (m/^$today:/io) { #dangerous -- has a gotcha
 
      }
  }
}

sub CheckLogfileForToday()
{
  my $today = (qw<Sun Mon Tue Wed Thu Fri Sat>)[(localtime)[6]];

  my $RegexObj = qr/^$today:/i; # compiles once per function call

  while (<LOGFILE>) {
      if ($_ =~ $RegexObj) {
 
      }
  }
}

if ($_ =~ $RegexObj) {

if (m/$RegexObj/) {

sub CheckLogfileForToday()
{
  my $today = (qw<Sun Mon Tue Wed Thu Fri Sat>)[(localtime)[6]];

  # Keep trying until one matches, so the default regex is set.
  "Sun:" =~ m/^$today:/i or
  "Mon:" =~ m/^$today:/i or
  "Tue:" =~ m/^$today:/i or
  "Wed:" =~ m/^$today:/i or
  "Thu:" =~ m/^$today:/i or
  "Fri:" =~ m/^$today:/i or
  "Sat:" =~ m/^$today:/i;

  while (<LOGFILE>) {
      if (m//) { # Now use the default regex
 
      }
  }
}

$Subject =~ s/^(?:Re:\s*)+//;

if ($Subject =~ m/^SPAM:(.+)/i) {
    $Subject = "-- spam subject removed --";
    $SpamCount{$1}++;
}

use English '-no_match_vars';

END {
   require Devel::SawAmpersand;
   if (Devel::SawAmpersand::sawampersand) {
       print "Naughty variable was used!\n";
   }
 }

use Time::HiRes;
sub CheckNaughtiness()
{
  my $text = 'x' x 10_000; # Create some non-small amount of data.

  # Calculate the overhead of a do-nothing loop.
  my $start = Time::HiRes::time();
  for (my $i = 0; $i < 5_000; $i++)  {  }
  my $overhead = Time::HiRes::time() - $start;

  # Now calculate the time for the same number of simple matches.
  $start = Time::HiRes::time();
  for (my $i = 0; $i < 5_000; $i++)  { $text =~ m/^/  }
  my $delta = Time::HiRes::time() - $start;

  # A differential of 5 is just a heuristic.
  printf "It seems your code is %s (overhead=%.2f, delta=%.2f)\n",
    ($delta > $overhead*5) ? "naughty" : "clean", $overhead, $delta;
}

while (<>)
{
   study($_); # Study the default target $_ before doing lots of matches on it
   if (m/regex 1/) { ... }
   if (m/regex 2/) { ... }
   if (m/regex 3/) { ... }
   if (m/regex 4/) { ... }
}

use Time::HiRes 'time';

my $start = time;
  
my $delta = time - $start;
printf "took %.1f seconds\n", $delta;

 % perl -cw -Mre=debug -e 'm/^Subject: (.*)/'
 Compiling REx `^Subject: (.*)'
 rarest char j at 3
    1: BOL(2)
    2: EXACT <Subject: >(6)
          
   12: END(0)
 anchored `Subject: ' at 0 (checking anchored) anchored(BOL) minlen 9
 Omitting $` $& $' support.

% perl -w -Mre=debug -e '"this is a test" =~ m/^Subject:/;'
    
Did not find anchored substr `Subject:'...
Match rejected by optimizer

% perl -w -Mre=debug -e 'use warnings'
 . . . lots of debugging information . . . 

java.util.regex.Pattern
java.util.regex.Matcher
java.util.regex.MatchResult
java.util.regex.PatternSyntaxException

public class SimpleRegexTest {
  public static void main(String[] args)
  {
     String myText = "this is my 1st test string";
     String myRegex = "\\d+\\w+";  // This provides for  「\d+\w+」
     java.util.regex.Pattern p = java.util.regex.Pattern.compile(myRegex);
     java.util.regex.Matcher m = p.matcher(myText);
     if (m.find()) {
         String matchedText = m.group();
         int    matchedFrom = m.start();
         int    matchedTo   = m.end();
         System.out.println("matched [" + matchedText + "] " +
                            "from " + matchedFrom +
                            " to " + matchedTo + ".");
     } else {
         System.out.println("didn't match");
     }
  }
}

import java.util.regex.*;

Pattern pat = Pattern.compile(myRegex,
                     Pattern.CASE_INSENSITIVE | Pattern.UNICODE_CASE);

Pattern.UNIX_LINES | Pattern.CASE_INSENSITIVE

String regex = "\\w+"; // 「\w+」
String text  = "Mastering Regular Expressions";
Matcher m = Pattern.compile(regex).matcher(text);
if (m.find())
   System.out.println("match [" + m.group() + "]");

match [Mastering]

while (m.find())
   System.out.println("match [" + m.group() + "]");

match [Mastering]
match [Regular]
match [Expressions]

"1234".matches("\\d+"); // true
"123!".matches("\\d+"); // false

String url   = "http://regex.info/blog";
String regex = "(?x) ^(https?):// ([^/:]+) (?:(\\d+))?";
Matcher m = Pattern.compile(regex).matcher(url);

if (m.find())
{
  System.out.print(
      "Overall  [" + m.group()  + "]" +
      " (from "    + m.start()  + " to " + m.end()  + ")\n" +

      "Protocol [" + m.group(1) + "]" +
      " (from "    + m.start(1) + " to " + m.end(1) + ")\n" +

      "Hostname [" + m.group(2) + "]" +
      " (from "    + m.start(2) + " to " + m.end(2) + ")\n"

  );

  // Group #3 might not have participated, so we must be careful here
  if (m.group(3) == null)
     System.out.println("No port; default of '80' is assumed");
  else  {
     System.out.print("Port is [" + m.group(3) + "] " +
                      "(from " + m.start(3) + " to " + m.end(3) + ")\n");

  }

}

Overall  [http://regex.info] (from 0 to 17)
Protocol [http] (from 0 to 4)
Hostname [regex.info] (from 7 to 17)
No port; default of '80' is assumed

string.replaceAll(regex, replacement)

Pattern.compile(regex).matcher(string).replaceAll(replacement)

String text = "Before Java 1.5 was Java 1.4.2. After Java 1.5 is Java 1.6";
String regex = "\\bJava\\s*1\\.5\\b";
Matcher m = Pattern.compile(regex).matcher(text);
String result = m.replaceAll("Java 5.0");
System.out.println(result);

Before Java 5.0 was Java 1.4.2. After Java 5.0 is Java 1.6

Pattern.compile("\\bJava\\s*1\\.5\\b").matcher(text).replaceAll("Java 5.0")

Pattern.compile("\\bJava\\s*1\\.([56])\\b").matcher(text).replaceAll("Java $1.0")

Before Java 5.0 was Java 1.4.2. After Java 5.0 is Java 6.0

Pattern.compile(uRegex).matcher(text).replaceAll(Matcher.quoteReplacement(uRepl))

while (m.find())
    m.appendReplacement(sb, "XXX")

m.appendTail(sb)

public static String replaceAll(Matcher m, String replacement)
{
   m.reset(); // Be sure to start with a fresh Matcher object
   StringBuffer result = new StringBuffer(); // We'll build the updated copy here

   while (m.find())
       m.appendReplacement(result, replacement);

   m.appendTail(result);
   return result.toString(); // Convert result  to a string and return
}

public static String replaceAllRegion(Matcher m, String replacement)
{
   Integer start = m.regionStart();
   Integer end   = m.regionEnd();
   m.reset().region(start, end); // Reset the matcher, but then restore the region

   StringBuffer result = new StringBuffer(); // We'll build the updated copy here

   while (m.find())
       m.appendReplacement(result, replacement);

   m.appendTail(result);
   return result.toString(); // Convert to a String and return
}

// Build a matcher to find numbers followed by "C" within the variable "Metric"
// The following regex is: 「(\d+(?:\.\d*)?)C\b」
Matcher m = Pattern.compile("(\\d+(?:\\.\\d*)?)C\\b").matcher(metric);
StringBuffer result = new StringBuffer(); // We'll build the updated copy here

while (m.find())
{
  float celsius = Float.parseFloat(m.group(1));  // Get the number, as a number
  int fahrenheit = (int) (celsius * 9/5 + 32);   // Convert to a Fahrenheit value
  m.appendReplacement(result, fahrenheit + "F"); // Insert it
}

m.appendTail(result);
System.out.println(result.toString()); // Display the result

StringBuilder text = new StringBuilder("It's SO very RUDE to shout!");
Matcher m = Pattern.compile("\\b[\\p{Lu}\\p{Lt}]+\\b").matcher(text);

while (m.find())
   text.replace(m.start(), m.end(), m.group().toLowerCase());

System.out.println(text);

It's so very rude to shout!

StringBuilder text = new StringBuilder("It's SO very RUDE to shout!");
Matcher m = Pattern.compile("\\b[\\p{Lu}\\p{Lt}]+\\b").matcher(text);

int matchPointer = 0;// First search begins at the start of the string
while (m.find(matchPointer)) {
   matchPointer = m.end(); // Next search starts from where this one ended
   text.replace(m.start(), m.end(), "<b>"+ m.group().toLowerCase() +"</b>");
   matchPointer += 7; // Account for having added '<b>' and '</b>'
}

System.out.println(text);

It's <b>so</b> very <b>rude</b> to shout!

// Matcher to find an image tag. The 'html' variable contains the HTML in question
Matcher mImg = Pattern.compile("(?id)<IMG\\s+(.*?)/?>").matcher(html);

// Matcher to find an ALT attribute (to be applied to an IMG tag's body within the same 'html' variable)
Matcher mAlt = Pattern.compile("(?ix)\\b ALT \\s* =").matcher(html);

// For each image tag within the html . . . 
while (mImg.find()) {
   // Restrict the next ALT search to the body of the just-found image tag
   mAlt.region( mImg.start(1), mImg.end(1) );

   // Report an error if no ALT found, showing the whole image tag found above
   if (! mAlt.find())
       System.out.println("Missing ALT attribute in: " + mImg.group());
}

// Matcher to find an image tag. The 'html' variable contains the HTML in question
Matcher mImg  = Pattern.compile("(?id)<IMG\\s+(.*?)/?>").matcher(html);
// Matcher to find an ALT attribute (to be applied to an IMG tag's body within the same 'html' variable)
Matcher mAlt  = Pattern.compile("(?ix)\\b ALT \\s* =").matcher(html);

// Matcher to find a newline
Matcher mLine = Pattern.compile("\\n").matcher(html);

// For each image tag within the html . . . 
while (mImg.find())
{

   // Restrict the next ALT search to the body of the just-found image tag
   mAlt.region( mImg.start(1), mImg.end(1) );
   // Report an error if no ALT found, showing the whole image tag found above
   if (! mAlt.find()) {

      // Restrict counting of newlines to the text before the start of the image tag
      mLine.region(0, mImg.start());

      int lineNum = 1; // The first line is numbered 1
      while (mLine.find())
              lineNum++; // Each newline bumps up the line number

      System.out.println("Missing ALT attribute on line " + lineNum);
   }

}

Madagascar is much too large to see on foot, so you'll need a car.

String regex = "\\bcar\\b"; // 「\bcar\b」
String text  = "Madagascar is best seen by car or bike.";
Matcher m = Pattern.compile(regex).matcher(text);
m.region(7, text.length());
m.find();
System.out.println("Matches starting at character " + m.start());

Matches starting at character 7

m.useTransparentBounds(true);

Matches starting at character 27

Pattern p = Pattern.compile(regex); // Compile regex.
Matcher m = p.matcher(text);        // Associate regex with text, creating a Matcher.
m.region(5, text.length());         // Bump start of region five characters forward.
m.useAnchoringBounds(false);        // Don't let  「^」  et al. match at the region start.
m.useTransparentBounds(true);       // Let looking constructs see across region edges.

Matcher m = Pattern.compile(regex).matcher(text);
m.region(5, text.length());   // Bump start of region five characters forward.
m.useAnchoringBounds(false);  // Don't let  「^」  et al. match at the region start.
m.useTransparentBounds(true); // Let looking constructs see across region edges.

Matcher m = Pattern.compile(regex).matcher(text).region(5,text.length())
              .useAnchoringBounds(false).useTransparentBounds(true);

Matcher m = Pattern.compile("(\\w+)").matcher("ABC 123");
System.out.println(m.toString());
m.find();
System.out.println(m.toString());

java.util.regex.Matcher[pattern=(\w+) region=0,7 lastmatch=]
java.util.regex.Matcher[pattern=(\w+) region=0,7 lastmatch=ABC]

// This pattern, used in the function below, is compiled and saved here for efficiency.
static final Pattern pNeverFail = Pattern.compile("^");

// Return the target text associated with a matcher object.
public static String text(Matcher m)
{

   // Remember these items so that we can restore them later.
   Integer regionStart = m.regionStart();
   Integer regionEnd   = m.regionEnd();
   Pattern pattern     = m.pattern();

   // Fetch the string the only way the class allows.
   String text = m.usePattern(pNeverFail).replaceFirst("");

   // Put back what we changed (or might have changed).
   m.usePattern(pattern).region(regionStart, regionEnd);

   // Return the text
   return text;

}

Pattern.compile(regex).matcher(text).matches();

String[] result = Pattern.compile("\\.").split("209.204.146.22");

String[] result = Pattern.compile("\\W+").split(Text);

String[] result = Pattern.compile("\\s*,\\s*").split(", one, two , ,, 3");

String[] result = Pattern.compile(":").split(":xx:");

String[] result = Pattern.compile(":").split(":xx:", -1);

Friedl,Jeffrey,Eric Francis,America,Ohio,Rootstown

String[] NameInfo = Pattern.compile(",").split(Text, 4);
// NameInfo[0] is the family name.
// NameInfo[1] is the given name.
// NameInfo[2] is the middle name (or in my case, middle names).
// NameInfo[3] is everything else, which we don't need, so we'll just ignore it.

// Matcher for isolating <img> tags
Matcher mImg    = Pattern.compile("(?id)<IMG\\s+(.*?)/?>").matcher(html);

// Matchers that isolate the SRC, WIDTH, and HEIGHT attributes within a tag (with very naïve regexes)
Matcher mSrc    = Pattern.compile("(?ix)\\bSRC   =(\\S+)").matcher(html);
Matcher mWidth  = Pattern.compile("(?ix)\\bWIDTH =(\\S+)").matcher(html);
Matcher mHeight = Pattern.compile("(?ix)\\bHEIGHT=(\\S+)").matcher(html);

int imgMatchPointer = 0; // The first search begins at the start of the string
while (mImg.find(imgMatchPointer))
{

   imgMatchPointer = mImg.end(); // Next image search starts from where this one ended

   // Look for our attributes within the body of the just-found image tag
   Boolean hasSrc    =    mSrc.region( mImg.start(1), mImg.end(1) ).find();
   Boolean hasHeight = mHeight.region( mImg.start(1), mImg.end(1) ).find();
   Boolean hasWidth  =  mWidth.region( mImg.start(1), mImg.end(1) ).find();

   // If we have a SRC attribute, but are missing WIDTH and/or HEIGHT . . . 
   if (hasSrc && (! hasWidth || ! hasHeight))
   {

      java.awt.image.BufferedImage i = // this fetches the image
         javax.imageio.ImageIO.read(new java.net.URL(mSrc.group(1)));

      String size; // Will hold the missing WIDTH and/or HEIGHT attributes
      if (hasWidth)
          // We're told the width, so compute the height that maintains the proper aspect ratio
          size = "height='" + (int)(Integer.parseInt(mWidth.group(1)) *
                                    i.getHeight() / i.getWidth())  + "' ";
      else if (hasHeight)
          // We're told the height, so compute the width that maintains the proper aspect ratio
          size = "width='"  + (int)(Integer.parseInt(mHeight.group(1)) *
                                    i.getWidth()  / i.getHeight()) + "' ";
      else // We're told neither, so just insert the actual size
          size = "width='"  + i.getWidth()  + "' " +
                 "height='" + i.getHeight() + "' ";

      html.insert(mImg.start(1), size); // Update the HTML in place
      imgMatchPointer += size.length(); // Account for the new text in mImg's eyes

   }

}

Pattern pAtEnd   = Pattern.compile("\\G\\z");
Pattern pWord    = Pattern.compile("\\G\\w+");
Pattern pNonHtml = Pattern.compile("\\G[^\\w<>&]+");
Pattern pImgTag  = Pattern.compile("\\G(?i)<img\\s+([^>]+)>");
Pattern pLink    = Pattern.compile("\\G(?i)<A\\s+([^>]+)>");
Pattern pLinkX   = Pattern.compile("\\G(?i)</A>");
Pattern pEntity  = Pattern.compile("\\G&(#\\d+|\\w+);");

Boolean needClose = false;
Matcher m = pAtEnd.matcher(html); // Any Pattern object can create our Matcher object

while (! m.usePattern(pAtEnd).find())
{
   if (m.usePattern(pWord).find()) {
       . . . have a word or number in m.group() -- can now check for profanity, etc . . . 
   } else if (m.usePattern(pImgTag).find()) {
       . . . have an image tag -- can check that it's appropriate . . . 
   } else if (! needClose && m.usePattern(pLink).find()) {
       . . . have a link anchor -- can validate it . . . 
      needClose = true;
   } else if (needClose && m.usePattern(pLinkX).find()) {
      System.out.println("/LINK [" + m.group() + "]");
      needClose = false;
   } else if (m.usePattern(pEntity).find()) {
      // Allow entities like &gt; and &#123;
   } else if (m.usePattern(pNonHtml).find()) {
      // Other (non-word) non-HTML stuff -- simply allow it
   } else {
      // Nothing matched at this point, so it must be an error. Grab a dozen or so characters
      // at our current location so that we can issue an informative error message
      m.usePattern(Pattern.compile("\\G(?s).{1,12}")).find();
      System.out.println("Bad char before '" + m.group() + "'");
      System.exit(1);

   }

}

if (needClose) {
   System.out.println("Missing Final </A>");
   System.exit(1);

}

Pattern pWord    = Pattern.compile("\\G\\w+");
Pattern pNonHtml = Pattern.compile("\\G[^\\w<>&]+");
Pattern pImgTag  = Pattern.compile("\\G(?i)<img\\s+([^>]+)>");
Pattern pLink    = Pattern.compile("\\G(?i)<A\\s+([^>]+)>");
Pattern pLinkX   = Pattern.compile("\\G(?i)</A>");
Pattern pEntity  = Pattern.compile("\\G&(#\\d+|\\w+);");
Boolean needClose = false;
Matcher m = pWord.matcher(html);  // Any Pattern object can create our Matcher object
Integer currentLoc = 0;           // Begin at the start of the string

while (currentLoc < html.length())
{

   if (m.usePattern(pWord).find(currentLoc)) {
       . . . have a word or number in m.group() -- can now check for profanity, etc . . . 
   } else if (m.usePattern(pNonHtml).find(currentLoc)) {
      // Other (non-word) non-HTML stuff -- simply allow it
   } else if (m.usePattern(pImgTag).find(currentLoc)) {
       . . . have an image tag -- can check that it's appropriate . . . 
   } else if (! needClose && m.usePattern(pLink).find(currentLoc)) {
       . . . have a link anchor -- can validate it . . . 
      needClose = true;
   } else if (needClose && m.usePattern(pLinkX).find(currentLoc)) {
      System.out.println("/LINK [" + m.group() + "]");
      needClose = false;
   } else if (m.usePattern(pEntity).find(currentLoc)) {
      // Allow entities like &gt; and &#123;
   } else {
      // Nothing matched at this point, so it must be an error. Grab a dozen or so characters
      // at our current location so that we can issue an informative error message
      m.usePattern(Pattern.compile("\\G(?s).{1,12}")).find(currentLoc);
      System.out.println("Bad char at '" + m.group() + "'");
      System.exit(1);

   }
   currentLoc = m.end(); // The `current location' is now where the previous match ended

}

if (needClose) {
   System.out.println("Missing Final </A>");
   System.exit(1);

}

m.usePattern(pWord).region(start,end).find(currentLoc)

String regex = // Puts a double quoted field into group(1), an unquoted field into group(2).
    "  \\G(?:^|,)                                    \n"+
    "  (?:\n"+
    "       # Either a double-quoted field . . . \n"+
    "       \" # field's opening quote\n"+
    "        (   [^\"]*+  (?: \"\" [^\"]*+ )*+  )\n"+
    "       \" # field's closing quote\n"+
    "   |# . . . or . . . \n"+
    "       # some non-quote/non-comma text . . . \n"+
    "       ( [^\",]*+ )\n"+
    "  )\n";

// Create a matcher for the CSV line of text, using the regex above.
Matcher mMain = Pattern.compile(regex, Pattern.COMMENTS).matcher(line);

// Create a matcher for  「"" , with dummy text for the time being.
Matcher mQuote = Pattern.compile("\"\"").matcher("");

while (mMain.find())
{
    String field;
    if (mMain.start(2) >= 0)
        field = mMain.group(2); // The field is unquoted, so we can use it as is.
    else
        // The field is quoted, so we must replace paired double quotes with one double quote.
        field = mQuote.reset(mMain.group(1)).replaceAll("\"");

    // We can now work with field . . . 
    System.out.println("Field [" + field + "]");
}

(s1;\w)s1;(s1;?<Num>\d+)s1;(s1;\s+)s1;

RegexOptions.IgnoreCase
RegexOptions.Multiline
RegexOptions.Compiled

Imports System.Text.RegularExpressions

If Regex.IsMatch(TestStr, "^\s*$")
   Console.WriteLine("line is empty")
Else
   Console.WriteLine("line is not empty")
End If

If Regex.IsMatch(TestStr, "^subject:", RegexOptions.IgnoreCase)
   Console.WriteLine("line is a subject line")
Else
   Console.WriteLine("line is not a subject line")
End If

Dim TheNum as String = Regex.Match(TestStr, "\d+").Value
If TheNum <> ""
   Console.WriteLine("Number is: " & TheNum)
End If

Dim ImgTag as String = Regex.Match(TestStr, "<img\b[^>]*>", _
                                   RegexOptions.IgnoreCase).Value
If ImgTag <> ""
   Console.WriteLine("Image tag: " & ImgTag)
End If

Dim Subject as String = _
    Regex.Match(TestStr, "^Subject: (.*)").Groups(1).Value
If Subject <> ""
   Console.WriteLine("Subject is: " & Subject)
End If

Dim Subject as String = _
    Regex.Match(TestStr, "^subject: (.*)", _
                RegexOptions.IgnoreCase).Groups(1).Value
If Subject <> ""
   Console.WriteLine("Subject is: " & Subject)
End If

Dim Subject as String = _
    Regex.Match(TestStr, "^subject: (?<Subj>.*)", _
                 RegexOptions.IgnoreCase).Groups("Subj").Value
If Subject <> ""
   Console.WriteLine("Subject is: " & Subject)
End If

TestStr = Regex.Replace(TestStr, "&", "&")
TestStr = Regex.Replace(TestStr, "<", "&lt;")
TestStr = Regex.Replace(TestStr, ">", "&gt;")
Console.WriteLine("Now safe in HTML: " & TestStr)

TestStr = Regex.Replace(TestStr, "\b[A-Z]\w*", "<B>$&</B>")
Console.WriteLine("Modified string: " & TestStr)

TestStr = Regex.Replace(TestStr, "<b>(.*?)</b>", "<I>$1</I>", _
                        RegexOptions.IgnoreCase)
Console.WriteLine("Modified string: " & TestStr)

Option Explicit On ' These are not specifically required to use regexes,
Option Strict On   ' but their use is good general practice.

' Make regex-related classes easily available.
Imports System.Text.RegularExpressions

Module SimpleTest
Sub Main()
     Dim SampleText as String = "this is the 1st test string"
     Dim R as Regex = New Regex("\d+\w+") 'Compile the pattern.
     Dim M as Match = R.match(SampleText) 'Check against a string.
     If not M.Success
         Console.WriteLine("no match")
     Else
         Dim MatchedText as String  = M.Value 'Query the results . . . 
         Dim MatchedFrom as Integer = M.Index
         Dim MatchedLen  as Integer = M.Length
         Console.WriteLine("matched [" & MatchedText & "]" & _
                           " from char#" & MatchedFrom.ToString() & _
                           " for " & MatchedLen.ToString() & " chars.")
     End If
End Sub
End Module

using System.Text.RegularExpressions; // This is for C#

Dim R as Regex = New Regex("\d+\w+") 'Compile the pattern.
Dim M as Match = R.Match(SampleText) 'Check against a string.

Dim M as Match = Regex.Match(SampleText, "\d+\w+") 'Check pattern against string.

Option Explicit On
Option Strict On
Imports System.Text.RegularExpressions

Dim R as Regex = New Regex("\s+(\d+)")

Dim M as Match = R.Match("May 16, 1998")

Dim StripTrailWS = new Regex("\s+$") ' for removing trailing whitespace

Dim GetSubject = new Regex("^subject: (.*)", RegexOptions.IgnoreCase)

Dim GetSubject = new Regex("^subject: (.*)", _
                   RegexOptions.IgnoreCase OR RegexOptions.Multiline)

Dim R As Regex
Try
    R = New Regex(SearchRegex)
Catch e As ArgumentException
    Console.WriteLine("*ERROR* bad regex: " & e.ToString)
    Exit Sub
End Try

Dim R as Regex = New Regex( _
   "# Match a floating-point number ...           " & chr(10) & _
   "  \d+(?:\.\d*)? # with a leading digit...     " & chr(10) & _
   "  |             # or ...                      " & chr(10) & _
   "  \.\d+         # with a leading decimal point", _
   RegexOptions.IgnorePatternWhitespace)

Dim R as Regex = New Regex( _
   "(?# Match a floating-point number ...            )" & _
   "  \d+(?:\.\d*)? (?# with a leading digit...      )" & _
   "  |             (?# or ...                       )" & _
   "  \.\d+         (?# with a leading decimal point )", _
   RegexOptions.IgnorePatternWhitespace)

Dim R as RegexObj = New Regex("^\s*$")
   
If R.IsMatch(Line) Then
   ' Line is blank . . . 
        
Endif

Dim R as New Regex("\w+")
Dim Target as String = "a few words"

Dim BunchOfMatches as MatchCollection = R.Matches(Target)
Dim I as Integer
For I = 0 to BunchOfMatches.Count - 1
    Dim MatchObj as Match = BunchOfMatches.Item(I)
    Console.WriteLine("Match: " & MatchObj.Value)
Next

Dim MatchObj as Match
For Each MatchObj in R.Matches(Target)
    Console.WriteLine("Match: " & MatchObj.Value)
Next

Dim MatchObj as Match = R.Match(Target)
While MatchObj.Success
    Console.WriteLine("Match: " & MatchObj.Value)
    MatchObj = MatchObj.NextMatch()
End While

Dim R_CapWord as New Regex("\b[A-Z]\w*")
  
Text = R_CapWord.Replace(Text, "<B>$0</B>")

Dim AnyWS as New Regex("\s+")
  
Target = AnyWS.Replace(Target, " ")

Dim AnyWS     as New Regex("\s+")
Dim LeadingWS as New Regex("^\s+")
  
Target = AnyWS.Replace(Target, " ", -1, LeadingWS.Match(Target).Length)

Target = R.Replace(Target, "<<$&>>"))
 
Function MatchFunc(ByVal M as Match) as String
  return M.Result("<<$&>>")
End Function
Dim Evaluator as MatchEvaluator = New MatchEvaluator(AddressOf MatchFunc)
   
Target = R.Replace(Target, Evaluator)

Function MatchFunc(ByVal M as Match) as String
  'Get numeric temperature from $1, then convert to Fahrenheit
  Dim Celsius as Double = Double.Parse(M.Groups(1).Value)
  Dim Fahrenheit as Double = Celsius * 9/5 + 32
  Return Fahrenheit & "F" 'Append an "F", and return
End Function

Dim Evaluator as MatchEvaluator = New MatchEvaluator(AddressOf MatchFunc)
  
Dim R_Temp as Regex = New Regex("(\d+)C\b", RegexOptions.IgnoreCase)
Target = R_Temp.Replace(Target, Evaluator)

Dim R as New Regex("\.")
Dim Parts as String() = R.Split("209.204.146.22")

Dim R as New Regex("\.")
Dim Parts as String() = R.Split("209.204.146.22", 2)

Dim R as New Regex("[-/]")
Dim Parts as String() = R.Split(MyDate)

'Display information known about the Regex object in the variable R
Console.WriteLine("Regex is: " & R.ToString())
Console.WriteLine("Options are: " & R.Options)
If R.RightToLeft
   Console.WriteLine("Is Right-To-Left: True")
Else
   Console.WriteLine("Is Right-To-Left: False")
End If

Dim S as String
For Each S in R.GetGroupNames()
    Console.WriteLine("Name """ & S & """ is Num #" & _
                      R.GroupNumberFromName(S))
Next
Console.WriteLine("---")
Dim I as Integer
For Each I in R.GetGroupNumbers()
    Console.WriteLine("Num #" & I & " is Name """ & _
                      R.GroupNameFromNumber(I) & """")
Next

New Regex("^(\w+)://([^/]+)(/\S*)")

New Regex("^(?<proto>\w+)://(?<host>[^/]+)(?<page>/\S*)",
          RegexOptions.Compiled)

Dim M as Match = Regex.Match(SomeString, "\w+")
Console.WriteLine(M.Result("The first word is '$&'"))

M.Result("$`") 'This is the text to the left of the match
M.Result("$'") 'This is the text to the right of the match

M.Result("[$`<$&>$']"))

Regex.IsMatch(target, pattern)
Regex.IsMatch(target, pattern, options)

Regex.Match(target, pattern)
Regex.Match(target, pattern, options)

Regex.Matches(target, pattern)
Regex.Matches(target, pattern, options)

Regex.Replace(target, pattern, replacement)
Regex.Replace(target, pattern, replacement, options)

Regex.Split(target, pattern)
Regex.Split(target, pattern, options)

If Regex.IsMatch(Line, "^\s*$")

Dim TemporaryRegex = New Regex("^\s*$")
If TemporaryRegex.IsMatch(Line)

If New Regex("^\s*$").IsMatch(Line)

Regex.CacheSize = 123

Dim UserRegex as Regex = New Regex("^" & Regex.Escape(SearchTerm) & "$", _
                                   RegexOptions.IgnoreCase)

Dim SubMatch as Match = Match.Empty 'Initialize, in case it's not set in the loop below
  
Dim Line as String
For Each Line in EmailHeaderLines
   'If this is the subject, save the match info for later . . . 
   Dim ThisMatch as Match = Regex.Match(Line, "^Subject:\s*(.*)", _
                                        RegexOptions.IgnoreCase)
   If ThisMatch.Success
      SubMatch = ThisMatch
   End If
     
Next
  
If SubMatch.Success
   Console.WriteLine(SubMatch.Result("The subject is: $1"))
Else
   Console.WriteLine("No subject!")
End If

Option Explicit On
Option Strict On

Imports System.Text.RegularExpressions
Imports System.Reflection

Module BuildMyLibrary
Sub Main()
 'The calls to RegexCompilationInfo below provide the pattern, regex options, name within the class,
 'class name, and a Boolean indicating whether the new class is public. The first class, for example,
 'will be available to programs that use this assembly as "jfriedl.Mail.Subject", a Regex constructor.
 Dim RCInfo() as RegexCompilationInfo = {                           _
   New RegexCompilationInfo(                                        _
     "^Subject:\s*(.*)", RegexOptions.IgnoreCase,                   _
     "Subject", "jfriedl.Mail", true),                              _
   New RegexCompilationInfo(                                        _
     "^From:\s*(.*)", RegexOptions.IgnoreCase,                      _
     "From", "jfriedl.Mail", true),                                 _
   New RegexCompilationInfo(                                        _
     "\G(?:^|,)                                    " &              _
     "(?:                                          " &              _
     "  (?# Either a double-quoted field... )      " &              _
     "  ""  (?# field's opening quote )            " &              _
     "   (?<QuotedField>  (?> [^""]+ | """" )*   ) " &              _
     "  ""  (?# field's closing quote )            " &              _
     " (?# ...or... )                              " &              _
     " |                                           " &              _
     "  (?# ...some non-quote/non-comma text... )  " &              _
     "  (?<UnquotedField> [^"",]* )                " &              _
     " )",                                                          _
     RegexOptions.IgnorePatternWhitespace,                          _
     "GetField", "jfriedl.CSV", true)                               _
 }
 'Now do the heavy lifting to build and write out the whole thing . . . 
 Dim AN as AssemblyName = new AssemblyName()
 AN.Name = "JfriedlsRegexLibrary" 'This will be the DLL's filename
 AN.Version = New Version("1.0.0.0")
 Regex.CompileToAssembly(RCInfo, AN) 'Build everything
End Sub
End Module

Imports jfriedl

Dim FieldRegex as CSV.GetField = New CSV.GetField 'This makes a new Regex object
    
Dim FieldMatch as Match = FieldRegex.Match(Line) 'Apply the regex to a string . . . 
While FieldMatch.Success
  Dim Field as String
  If FieldMatch.Groups(1).Success
    Field = FieldMatch.Groups("QuotedField").Value
    Field = Regex.Replace(Field, """""", """") 'replace two double quotes with one
  Else
    Field = FieldMatch.Groups("UnquotedField").Value
  End If

  Console.WriteLine("[" & Field & "]")
  ' Can now work with 'Field'....

  FieldMatch = FieldMatch.NextMatch
End While

Dim FieldRegex as GetField = New GetField 'This makes a new Regex object

Dim FieldRegex as jfriedl.CSV.GetField = New jfriedl.CSV.GetField

Dim R As Regex = New Regex(" \(                    " & _
                           "   (?>                 " & _
                           "       [^()]+          " & _
                           "     |                 " & _
                           "       \( (?<DEPTH>)   " & _
                           "     |                 " & _
                           "       \) (?<-DEPTH>)  " & _
                           "   )*                  " & _
                           "   (?(DEPTH)(?!))      " & _
                           " \)                    ",  _
       RegexOptions.IgnorePatternWhitespace)

Dim M as Match = Regex.Match("abcdefghijk", "^(..)+")

M.Groups(1).Captures( M.Groups(1).Captures.Count - 1 ).Value

/* Check whether HTML tag is a <table> tag */
if (preg_match('/^<table\b/i', $tag))
  print "tag is a table tag\n";

/* Check whether text is an integer */
if (preg_match('/^-?\d+$/', $user_input))
  print "user input is an integer\n";

/* Pluck HTML title from a string */
if (preg_match('{<title>(.*?)</title>}si', $html, $matches))
  print "page title: $matches[1]\n";

/* Treat numbers in string as Fahrenheit values and replace with Celsius values */
$metric = preg_replace('/(-?\d+(?:\.\d+)?)/e',     /* pattern */
                       'floor(($1-32)*5/9 + 0.5)', /* replacement code */
                       $string);

/* Create an array of values from a string filled with simple comma-separated values */
$values_array = preg_split('!\s*,\s*!', $comma_separated_values);

print '/^.*\/';        prints:  /^.*\/     
print '/^.*\\/';       prints:  /^.*\/     
print '/^.*\\\/';      prints:  /^.*\\/     
print '/^.*\\\\/';     prints:  /^.*\\/

{ ( < [

} ) > ]

if (preg_match('{<title>(.*?)</title>}si', $html, $captures))

preg_match('<(\w+)([^>]*)>', $html)

Warning: Unknown modifier ']'

preg_match('/<(\w+)(.*?)>/', $html)

Warning: preg_match(): Unknown modifier ']'

preg_match('<(\w+)(.*?)>', $html)

preg_match(pattern, subject [, matches [, flags [, offset ]]])

preg_match($pattern, $subject)

if (preg_match('/\.(jpe?g|png|gif|bmp)$/i', $url)) {
  /* URL seems to be of an image */
}

if (preg_match('{^https?://}', $uri)) {
  /* URI is http or https */
}

if (preg_match('/\b MSIE \b/x', $_SERVER['HTTP_USER_AGENT'])) {
  /* Browser is IE */
}

/* Given a full path, isolate the filename */
if (preg_match('{ / ([^/]+) $}x', $WholePath, $matches))
  $FileName = $matches[1];

/* Pluck the protocol, hostname, and port number from a URL */
if (preg_match('{^(https?):// ([^/:]+) (?::(\d+))? }x', $url, $matches))
{

  $proto = $matches[1];
  $host  = $matches[2];
  $port  = $matches[3] ? $matches[3] : ($proto == "http" ? 80 : 443);

  print "Protocol: $proto\n";
  print "Host    : $host\n";
  print "Port    : $port\n";

}

/* Pluck the protocol, hostname, and port number from a URL */
if (preg_match('{^(?P<proto> https?   ) ://
                  (?P<host>  [^/:]+  )
            (?: :(?P<port>  \d+     ) )?  }x', $url, $matches))
{

  $proto = $matches['proto'];
  $host  = $matches['host'];
  $port  = $matches['port'] ? $matches['port'] : ($proto=="http"?80:443);

  print "Protocol: $proto\n";
  print "Host    : $host\n";
  print "Port    : $port\n";

}

/* Pluck the protocol, hostname, and port number from a URL */
if (preg_match('{^(?P<proto> https? )://
                  (?P<host> [^/:]+  )
            (?: :(?P<port> \d+     )  )?  }x', $url, $UrlInfo))
{

  if (! $UrlInfo['port'])
     $UrlInfo['port'] = ($UrlInfo['proto'] == "http" ? 80 : 443);

  echo "Protocol: ", $UrlInfo['proto'], "\n";
  echo "Host    : ", $UrlInfo['host'], "\n";
  echo "Port    : ", $UrlInfo['port'], "\n";

}

array
(

    0       => 'http://regex.info',
    'proto' => 'http',
    1       => 'http',
    'host'  => 'regex.info',
    2       => 'regex.info'

)

preg_match('/href \s*=\s* (?: "([^"]*)" | \'([^\']*)\' | ([^\s\'">]+) )/ix',
           $tag,
           $matches,
           PREG_OFFSET_CAPTURE);

<a name=bloglink href='http://regex.info/blog/' rel="nofollow">

array
(

    /* Data for the overall match */
    0 => array ( 0 => "href='http://regex.info/blog/'",
                 1 => 17 ),

    /* Data for the first set of parentheses */
    1 => array ( 0 => "",
                 1 => -1 ),

    /* Data for the second set of parentheses */
    2 => array ( 0 => "http://regex.info/blog/",
                 1 => 23 )

)

substr($tag, $matches[0][1], strlen($matches[0][0]));

function reg_match($regex, $subject, &$matches, $offset = 0)
{
    $result = preg_match($regex, $subject, $matches,
                         PREG_OFFSET_CAPTURE, $offset);
    if ($result) {
        $f = create_function('&$X', '$X = $X[1] < 0 ?  NULL : $X[0];');
        array_walk($matches, $f);
    }
    return $result;
}

preg_match_all(pattern, subject, matches [, flags [, offset ]])

if (preg_match_all('/<title>/i', $html, $all_matches) > 1)
  print "whoa, document has more than one <title>!\n";

$subject = "
Jack A. Smith
Mary B. Miller";

/* No order-related flag implies PREG_PATTERN_ORDER */
preg_match_all('/^(\w+) (\w\.) (\w+)$/m', $subject, $all_matches);

array
(
  /* $all_matches[0] is an array of full matches */
  0 => array ( 0 => "Jack A. Smith",  /* full text from first match    */
               1 => "Mary B. Miller"  /* full text from second match */ ),

  /* $all_matches[1] is an array of strings captured by 1st set of parens */
  1 => array ( 0 => "Jack",  /* first match's 1st capturing parens */
               1 => "Mary"   /* second match's 1st capturing parens */ ),

  /* $all_matches[2] is an array of strings captured by 2nd set of parens */
  2 => array ( 0 => "A.",    /* first match's 2nd capturing parens */
               1 => "B."     /* second match's 2nd capturing parens */ ),

  /* $all_matches[3] is an array of strings captured by 3rd set of parens */
  3 => array ( 0 => "Smith", /* first match's 3rd capturing parens */
               1 => "Miller" /* second match's 3rd capturing parens */ )
)

$subject = "
Jack A. Smith
Mary B. Miller";preg_match_all('/^(\w+) (\w\.) (\w+)$/m', $subject, $all_matches, PREG_SET_ORDER);

array
(
  /* $all_matches[0] is just like a preg_match's entire $matches */
  0 => array ( 0 => "Jack A. Smith",  /* first match's full match           */
               1 => "Jack",          /* first match's 1st capturing parens*/
               2 => "A.",            /* first match's 2nd capturing parens*/
               3 => "Smith"          /* first match's 3rd capturing parens*/ ),

  /* $all_matches[1] is also just like a preg_match's entire $matches */
  1 => array ( 0 => "Mary B. Miller", /* second match's full match*/
               1 => "Mary",           /* second match's 1st capturing parens*/
               2 => "B.",             /* second match's 2nd capturing parens*/
               3 => "Miller"          /* second match's 3rd capturing parens*/ ),
)

preg_match_all($pattern, $subject, $all_matches,
               PREG_OFFSET_CAPTURE | PREG_SET_ORDER);

$subject = "
Jack A. Smith
Mary B. Miller";

/* No order-related flag implies PREG_PATTERN_ORDER */
preg_match_all('/^(?P<Given>\w+) (?P<Middle>\w\.) (?P<Family>\w+)$/m',
               $subject, $all_matches);

array
(

    0        => array ( 0 => "Jack A. Smith",  1 => "Mary B. Miller" ),

    "Given"  => array ( 0 => "Jack",    1 => "Mary"   ),
    1        => array ( 0 => "Jack",    1 => "Mary"   ),

    "Middle" => array ( 0 => "A.",      1 => "B."     ),
    2        => array ( 0 => "A.",      1 => "B."     ),

    "Family" => array ( 0 => "Smith",   1 => "Miller" ),
    3        => array ( 0 => "Smith",   1 => "Miller" )

)

$subject = "
Jack A. Smith
Mary B. Miller";

preg_match_all('/^(?P<Given>\w+) (?P<Middle>\w\.) (?P<Family>\w+)$/m',
               $subject, $all_matches, PREG_SET_ORDER);

array
(

    0 => array ( 0      => "Jack A. Smith",

                 Given  => "Jack",
                 1      => "Jack",

                 Middle => "A.",
                 2      => "A.",

                 Family => "Smith",
                 3      => "Smith" ),

    1 => array ( 0      => "Mary B. Miller",

                 Given  => "Mary",
                 1      => "Mary",

                 Middle => "B.",
                 2      => "B.",

                 Family => "Miller",
                 3      => "Miller" )

)

preg_replace(pattern, replacement, subject [, limit [, count ]])

$card_number = preg_replace('/\D+/', '', $card_number);
/* $card_number now has only digits, or is empty */

$html = preg_replace('/\b[A-Z]{2,}\b/', '<b>$0</b>', $html);

$html = preg_replace('/\b[A-Z]{2,}\b/e',
                     'strtolower("<b>$0</b>")',
                     $html);

$replacement = array ('&' => '&',
                      '<' => '&lt;',
                      '>' => '&gt;',
                      '"' => '&quot;');

$new_subject = preg_replace('/[&<">]/eS', '$replacement["$0"]', $subject);

$cooked = preg_replace(
   /* Match with these . . .   */ array('/&/',   '/</',  '/>/',  '/"/' ),
   /* Replace with these . . . */ array('&amp;', '&lt;', '&gt;', '&quot;'),
   /*  . . . in a copy of this*/ $text
);

AT&T --> "baby Bells"

AT&T --> "baby Bells"

$patterns     = array('/&/',   '/</',  '/>/',  '/"/' );
$replacements = array('&amp;', '&lt;', '&gt;', '&quot;');

$cooked = preg_replace($patterns, $replacements, $text);

$result_array = preg_replace($regex_array, $replace_array, $subject_array);

$result_array = array();

foreach ($subject_array as $subject)
{
   reset($regex_array);   // Prepare to walk through these two arrays
   reset($replace_array); // in their internal array orders.
   
   while (list(,$regex) = each($regex_array))
   {
       list(,$replacement) = each($replace_array);
       // The regex and replacemnet are ready, so apply to the subject . . . 
       $subject = preg_replace($regex, $replacement, $subject);
   }
   // Having now been processed by all the regexes, we're done with this subject . . . 
   $result_array[] = $subject;  // . . . so append to the results array.
}

$subject = "this has 7 words and 31 letters";

$result = preg_replace(array('/[a-z]+/', '/\d+/'),
                       array('word<$0>', 'num<$0>'),
                       $subject);

print "result: $result\n";

result: word<this> word<has> num<7> word<words> word<and> num<31> word<letters>

     $subject = "this has 7 words and 31 letters";

     $result = preg_replace(array('/\d+/',   '/[a-z]+/'),
                           array('num<\0>', 'word<\0>'),
                           $subject);

     print "result: $result\n";

result: word<this> word<has> word<num><7> word<words>
word<and> word<num><31> word<letters>

preg_replace_callback(pattern, callback, subject [, limit [, count ]])

   $replacement = array ('&' => '&',
                         '<' => '&lt;',
                         '>' => '&gt;',
                         '"' => '&quot;');

   /*
    * Given a $matches from a successful match in which $matches[0] is the text character in need of
    * conversion to HTML, return the appropriate HTML string. Because this function is used under only
    * carefully controlled conditions, we feel safe blindly using the arguments.
    */
   function text2html_callback($matches)
   {

       global $replacement;
       return $replacement[$matches[0]];

   }

   $new_subject = preg_replace_callback('/[&<">]/S',           /* pattern */
                                        "text2html_callback",/* callback */
                                        $subject);

"AT&T" sounds like "ATNT"

"AT&T" sounds like "ATNT"

$new_subject = preg_replace_callback('/[&<">]/S',
                 create_function('$matches',
                                 'global $replacement;
                                  return $replacement[$matches[0]];'),
                 $subject);

preg_split(pattern, subject [, limit, [ flags ]])

$tickers = explode(' ', $input);

$tickers = preg_split('/\s+/', $input);

$tickers = preg_split('/[\s,]+/', $input);

$tags = preg_split('/\s*,\s*/', $input);

$parts = preg_split('/\r? \n \r? \n/x', $response, 2);

$fields = preg_split('/\s* , \s*/x', $data, 3);

$tags = preg_split('/\s* , \s*/x', $input);

$tags = preg_split('/\s* , \s*/x', $input, -1, PREG_SPLIT_NO_EMPTY);

DLSR camera and Nikon D200 or Canon EOS 30D

$parts = preg_split('/\s+ (and|or) \s+/x', $input);

array ('DLSR camera', 'Nikon D200', 'Canon EOS 30D')

$parts = preg_split('/\s+ (and|or) \s+/x', $input, -1,
                    PREG_SPLIT_DELIM_CAPTURE);

array ('DLSR camera', 'and', 'Nikon D200', 'or', 'Canon EOS 30D')

$tickers = preg_split('/[\s,]+/', $input);

$tickers = preg_split('/([\s,]+)/', $input, -1, PREG_SPLIT_DELIM_CAPTURE);

preg_grep(pattern, input [, flags ])

preg_grep('/\s/', $input);

preg_grep('/\s/', $input, PREG_GREP_INVERT);

preg_grep('/^\S+$/', $input);

preg_quote(input [, delimiter ])

/* Given $MailSubject, find if $MailMessage is about that subject */
$pattern = '/^Subject:\s+(Re:\s*)*' . preg_quote($MailSubject, '/') . '/mi';

**Super Deal** (Act Now!)

/^Subject:\s+(Re:\s*)*\*\*Super Deal\*\* \(Act Now\!\)/mi

/*
 * Given a raw regex in a string (and, optionally, a pattern-modifiers string), return a string suitable
 * for use as a preg pattern. The regex is wrapped in delimiters, with the modifiers (if any) appended.
 */
function preg_regex_to_pattern($raw_regex, $modifiers = "")
{

  /*
   * To convert a regex to a pattern, we must wrap the pattern in delimiters (we'll use a pair of
   * forward slashes) and append the modifiers. We must also be sure to escape any unescaped
   * occurrences of the delimiter within the regex, and to escape a regex-ending escape
   * (which, if left alone, would end up escaping the delimiter we append).
   *
   * We can't just blindly escape embedded delimiters, because it would break a regex containing
   * an already-escaped delimiter. For example, if the regex is '\/', a blind escape results
   * in '\\/' which would not work when eventually wrapped with delimiters: '/\\//'.
   *
   * Rather, we'll break down the regex into sections: escaped characters, unescaped forward
   * slashes (which we'll need to escape), and everything else. As a special case, we also look out
   * for, and escape, a regex-ending escape.
   */
  if (! preg_match('{\\\\(?:/|$)}', $raw_regex)) /* '/' followed by '\' or EOS */
  {

     /* There are no already-escaped forward slashes, and no escape at the end, so it's
      * safe to blindly escape forward slashes. */
     $cooked = preg_replace('!/!', '\/', $raw_regex);

  }
  else
  {

     /* This is the pattern we'll use to parse $raw_regex.
      * The two parts whose matches we'll need to escape are within capturing parens. */
     $pattern = '{  [^\\\\/]+  |  \\\\.  |  (  /  |  \\\\$  )  }sx';

     /* Our callback function is called upon each successful match of $pattern in $raw-regex.
      * If $matches[1] is not empty, we return an escaped version of it.
      * Otherwise, we simply return what was matched unmodified. */
     $f = create_function('$matches', '     // This long
          if (empty($matches[1]))          // singlequoted
              return $matches[0];          // string becomes
          else                             // our function
              return "\\\\" . $matches[1]; // code.
     ');

     /* Actually apply $pattern to $raw_regex, yielding $cooked */
     $cooked = preg_replace_callback($pattern, $f, $raw_regex);

  }

  /* $cooked is now safe to wrap -- do so, append the modifiers, and return */
  return "/$cooked/$modifiers";
}

Compilation failed: nothing to repeat at offset 0

/*
 * Return an error message if the given pattern argument or its underlying regular expression
 * are not syntactically valid. Otherwise (if they are valid), false is returned.
 */
function preg_pattern_error($pattern)
{

   /*
    * To tell if the pattern has errors, we simply try to use it.
    * To detect and capture the error is not so simple, especially if we want to be sociable and not
    * tramp on global state (e.g., the value of $php_errormsg). So, if 'track_errors' is on, we preserve
    * the $php_errormsg value and restore it later. If 'track_errors' is not on, we turn it on (because
    * we need it) but turn it off when we're done.
    */
   if ($old_track = ini_get("track_errors"))
       $old_message = isset($php_errormsg) ? $php_errormsg : false; 
   else 
       ini_set('track_errors', 1);
   /* We're now sure that track_errors is on. */

   unset($php_errormsg);
   @ preg_match($pattern, ""); /* actually give the pattern a try! */
   $return_value = isset($php_errormsg) ? $php_errormsg : false;

   /* We've now captured what we need; restore global state to what it was. */
   if ($old_track)
       $php_errormsg = isset($old_message) ? $old_message : false;
   else
       ini_set('track_errors', 0);

   return $return_value;

}

/*
 * Return a descriptive error message if the given regular expression is invalid.
 * If it's valid, false is returned.
 */
function preg_regex_error($regex)
{
    return preg_pattern_error(preg_regex_to_pattern($regex));
}

if (preg_match('/^ (  (?: [^()]++ | \( (?1) \)  )*  ) $/x', $text))
   echo "text is balanced\n";
else
   echo "text is unbalanced\n";

$pattern = '{
    # The regular expression begins here . . . 
    ^
      (?P<stuff>
          # Everything within this set of parentheses is named "stuff."
          (?:
              [^()]++            # anything not parentheses
            |
              \(  (?P>stuff)  \) # an open paren, more "stuff," and finally a close paren.
          )*
      )
    $
    # This is the end of the regular expression.
}x'; # The 'x' here is a preg pattern modifier.

if (preg_match($pattern, $text))
   echo "text is balanced\n";
else
   echo "text is unbalanced\n";

$csv_regex = '{
     \G(?:^|,)
     (?:
        # Either a double-quoted field . . . 
        " # field opening quote
         (  [^"]*+  (?: "" [^"]*+ )*+  )
        " # field closing quote
      | # . . . or . . . 
        # . . . some non-quote/non-comma text . . . 
        ( [^",]*+ )
     )
  }x';

/* Apply the regex, filling $all_matches with all kinds of data */
preg_match_all($csv_regex, $line, $all_matches);

/* $Result will hold the array of fields we'll glean from $all_matches */
$Result = array ();

/* Run through each successful match . . .  */
for ($i = 0; $i < count($all_matches[0]); $i++)
{
  /* If the 2nd set of capturing parentheses captured, use that directly */
  if (strlen($all_matches[2][$i]) > 0)
     array_push($Result, $all_matches[2][$i]);
  else
  {
    /* It was a quoted value, so take care of an embedded double double-quote before using */
    array_push($Result, preg_replace('/""/', '"', $all_matches[1][$i]));
  }
}

/* The array $Result is now populated and available for use */

$xml_regex = '{
   ^(

      (?: <(\w++) [^>]*+ (?<!/)>  (?1)  </\2>  # matched pair of tags
        | [^<>]++                              # non-tag stuff
        | <\w[^>]*+/>                          # self-closing tag
        | <!--.*?-->                           # comment
        | <!\[CDATA\[.*?]]>                    # cdata block
        | <\?.*?\?>                            # processing instruction
        | <![A-Z].*?>                          # Entity declaration, etc.

      )*+

   )$

}sx';

if (preg_match($xml_regex, $xml_string))
     echo "block structure seems valid\n";
else
     echo "block structure seems invalid\n";

$html_regex = '{
   ^(

      (?: <(\w++) [^>]*+ (?<!/)> (?1) </\2>  # matched pair of tags
        | [^<>]++                            # non-tag stuff
        | <\w[^>]*+/>                        # self-closing tag
        | <!--.*?-->                         # comment
        | <script\b[^>]*>.*?</script>        # script block

      )*+

   )$

}isx';

if (preg_match($html_regex, $html_string))
     echo "block structure seems valid\n";
else
     echo "block structure seems invalid\n";