Editing Regular expression (section)

==Examples==
The specific syntax rules vary depending on the specific implementation, [[programming language]], or [[library (computing)|library]] in use. Additionally, the functionality of regex implementations can vary between [[software versioning|versions]].

Because regexes can be difficult to both explain and understand without examples, interactive websites for testing regexes are a useful resource for learning regexes by experimentation.
This section provides a basic description of some of the properties of regexes by way of illustration.

The following conventions are used in the examples.<ref name="clarify000">The character 'm' is not always required to specify a [[Perl]] match operation. For example, <code>m/[^abc]/</code> could also be rendered as <code>/[^abc]/</code>. The 'm' is only necessary if the user wishes to specify a match operation without using a forward-slash as the regex [[delimiter]]. Sometimes it is useful to specify an alternate regex delimiter in order to avoid "[[delimiter collision]]". See '[http://perldoc.perl.org/perlre.html perldoc perlre] {{Webarchive|url=https://web.archive.org/web/20091231010052/http://perldoc.perl.org/perlre.html |date=2009-12-31}}' for more details.</ref>

 metacharacter(s) ;; the metacharacters column specifies the regex syntax being demonstrated
 =~ m//           ;; indicates a regex ''match'' operation in Perl
 =~ s///          ;; indicates a regex ''substitution'' operation in Perl

These regexes are all Perl-like syntax. Standard [[#POSIX Basic Regular Expressions|POSIX]] regular expressions are different.

Unless otherwise indicated, the following examples conform to the [[Perl]] programming language, release 5.8.8, January 31, 2006. This means that other implementations may lack support for some parts of the syntax shown here (e.g. basic vs. extended regex, <code>\( \)</code> vs. <code>()</code>, or lack of <code>\d</code> instead of [[POSIX]] <code>[:digit:]</code>).

The syntax and conventions used in these examples coincide with that of other programming environments as well.<ref>E.g., see ''[[Java (programming language)|Java]] [[O'Reilly Media#In a Nutshell|in a Nutshell]]'', p. 213; ''[[Python (programming language)|Python]] Scripting for Computational Science'', p. 320; Programming [[PHP]], p. 106.</ref>

{| class="wikitable"
|-
! Meta&shy;character(s)
! Description
! Example<ref>All the if statements return a TRUE value</ref>
|-
! <code>.</code>
| Normally matches any character except a newline. <br/>Within square brackets the dot is literal.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/...../) {
  print "$string1 has length >= 5.\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
Hello World
 has length >= 5.
</syntaxhighlight>
|-
! <code>( )</code>
| Groups a series of pattern elements to a single element. <br/>When you match a pattern within parentheses, you can use any of <code>$1</code>, <code>$2</code>, ... later to refer to the previously matched pattern. Some implementations may use a backslash notation instead, like <code>\1</code>, <code>\2</code>.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/(H..).(o..)/) {
  print "We matched '$1' and '$2'.\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
We matched 'Hel' and 'o W'.
</syntaxhighlight>
|-
! <code>+</code>
| Matches the preceding pattern element one or more times.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/l+/) {
  print "There are one or more consecutive letter \"l\"'s in $string1.\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
There are one or more consecutive letter "l"'s in Hello World.
</syntaxhighlight>
|-
! <code>?</code>
| Matches the preceding pattern element zero or one time.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/H.?e/) {
  print "There is an 'H' and a 'e' separated by ";
  print "0-1 characters (e.g., He Hue Hee).\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
There is an 'H' and a 'e' separated by 0-1 characters (e.g., He Hue Hee).
</syntaxhighlight>
|-
! <code>?</code>
| Modifies the <code>*</code>, <code>+</code>, <code>?</code> or <code>{M,N}</code>'d regex that comes before to match as few times as possible.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/(l.+?o)/) {
  print "The non-greedy match with 'l' followed by one or ";
  print "more characters is 'llo' rather than 'llo Wo'.\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
The non-greedy match with 'l' followed by one or more characters is 'llo' rather than 'llo Wo'.
</syntaxhighlight>
|-
! <code>*</code>
| Matches the preceding pattern element zero or more times.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/el*o/) {
  print "There is an 'e' followed by zero to many ";
  print "'l' followed by 'o' (e.g., eo, elo, ello, elllo).\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
There is an 'e' followed by zero to many 'l' followed by 'o' (e.g., eo, elo, ello, elllo).
</syntaxhighlight>
|-
! <code>{M,N}</code>
| Denotes the minimum M and the maximum N match count.<br/>N can be omitted and M can be 0: <code>{M}</code> matches "exactly" M times; <code>{M,}</code> matches "at least" M times; <code>{0,N}</code> matches "at most" N times.<br/><code>x* y+ z?</code> is thus equivalent to <code>x{0,} y{1,} z{0,1}</code>.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/l{1,2}/) {
  print "There exists a substring with at least 1 ";
  print "and at most 2 l's in $string1\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
There exists a substring with at least 1 and at most 2 l's in Hello World
</syntaxhighlight>
|-
! <code>[…]</code>
| Denotes a set of possible character matches.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/[aeiou]+/) {
  print "$string1 contains one or more vowels.\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
Hello World
 contains one or more vowels.
</syntaxhighlight>
|-
! <code><nowiki>|</nowiki></code>
| Separates alternate possibilities.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/(Hello|Hi|Pogo)/) {
  print "$string1 contains at least one of Hello, Hi, or Pogo.";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
Hello World
 contains at least one of Hello, Hi, or Pogo.
</syntaxhighlight>
|-
! <code>\b</code>
| Matches a zero-width boundary between a word-class character (see next) and either a non-word class character or an edge; same as
<code>(^\w|\w$|\W\w|\w\W)</code>.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/llo\b/) {
  print "There is a word that ends with 'llo'.\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
There is a word that ends with 'llo'.
</syntaxhighlight>
|-
! <code>\w</code>
| Matches an alphanumeric character, including "_"; <br/>same as <code>[A-Za-z0-9_]</code> in ASCII, and
: <code>[\p{Alphabetic}<wbr/>\p{GC=Mark}<wbr/>\p{GC=Decimal_Number}<wbr/>\p{GC=Connector_Punctuation}]</code>
in Unicode,<ref name="unicode"/> where the <code>Alphabetic</code> property contains more than Latin letters, and the <code>Decimal_Number</code> property contains more than Arab digits.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/\w/) {
  print "There is at least one alphanumeric ";
  print "character in $string1 (A-Z, a-z, 0-9, _).\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
There is at least one alphanumeric character in Hello World
 (A-Z, a-z, 0-9, _).
</syntaxhighlight>
|-
! <code>\W</code>
| Matches a ''non''-alphanumeric character, excluding "_"; <br/>same as <code>[^A-Za-z0-9_]</code> in ASCII, and
: <code>[^\p{Alphabetic}<wbr/>\p{GC=Mark}<wbr/>\p{GC=Decimal_Number}<wbr/>\p{GC=Connector_Punctuation}]</code>
in Unicode.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/\W/) {
  print "The space between Hello and ";
  print "World is not alphanumeric.\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
The space between Hello and World is not alphanumeric.
</syntaxhighlight>
|-
! <code>\s</code>
| Matches a whitespace character, <br/>which in ASCII are tab, line feed, form feed, carriage return, and space; <br/>in Unicode, also matches no-<wbr/>break spaces, next line, and the variable-<wbr/>width spaces (among others).
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/\s.*\s/) {
  print "In $string1 there are TWO whitespace characters, which may";
  print " be separated by other characters.\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
In Hello World
 there are TWO whitespace characters, which may be separated by other characters.
</syntaxhighlight>
|-
! <code>\S</code>
| Matches anything ''but'' a whitespace.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/\S.*\S/) {
  print "In $string1 there are TWO non-whitespace characters, which";
  print " may be separated by other characters.\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
In Hello World
 there are TWO non-whitespace characters, which may be separated by other characters.
</syntaxhighlight>
|-
! <code>\d</code>
| Matches a digit; <br/>same as <code>[0-9]</code> in ASCII; <br/>in Unicode, same as the <code>\p{Digit}</code> or <code>\p{GC=Decimal_Number}</code> property, which itself the same as the <code>\p{Numeric_Type=Decimal}</code> property.
| <syntaxhighlight lang="perl">
$string1 = "99 bottles of beer on the wall.";
if ($string1 =~ m/(\d+)/) {
  print "$1 is the first number in '$string1'\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
99 is the first number in '99 bottles of beer on the wall.'
</syntaxhighlight>
|-
! <code>\D</code>
| Matches a non-digit; <br/>same as <code>[^0-9]</code> in ASCII or <code>\P{Digit}</code> in Unicode.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/\D/) {
  print "At least one character in $string1";
  print " is not a digit.\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
At least one character in Hello World
 is not a digit.
</syntaxhighlight>
|-
! <code>^</code>
| Matches the beginning of a line or string.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/^He/) {
  print "$string1 starts with the characters 'He'.\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
Hello World
 starts with the characters 'He'.
</syntaxhighlight>
|-
! <code>$</code>
| Matches the end of a line or string.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/rld$/) {
  print "$string1 is a line or string ";
  print "that ends with 'rld'.\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
Hello World
 is a line or string that ends with 'rld'.
</syntaxhighlight>
|-
! <code>\A</code>
| Matches the beginning of a string (but not an internal line).
| <syntaxhighlight lang="perl">
$string1 = "Hello\nWorld\n";
if ($string1 =~ m/\AH/) {
  print "$string1 is a string ";
  print "that starts with 'H'.\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
Hello
World
 is a string that starts with 'H'.
</syntaxhighlight>
|-
! <code>\z</code>
| Matches the end of a string (but not an internal line).<ref name="Perl Best Practices">{{cite book |last=Conway |first=Damian |author-link=Damian Conway |title=Perl Best Practices |chapter=Regular Expressions, End of String |publisher=[[O'Reilly Media|O'Reilly]] |page=240 |chapter-url=https://www.scribd.com/doc/15491004/Perl-Best-Practices |year=2005 |isbn=978-0-596-00173-5 |access-date=2017-09-10 |archive-date=2020-10-07 |archive-url=https://web.archive.org/web/20201007183212/https://www.scribd.com/book/15491004/Perl-Best-Practices-Standards-and-Styles-for-Developing-Maintainable-Code |url-status=live}}</ref>
| <syntaxhighlight lang="perl">
$string1 = "Hello\nWorld\n";
if ($string1 =~ m/d\n\z/) {
  print "$string1 is a string ";
  print "that ends with 'd\\n'.\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
Hello
World
 is a string that ends with 'd\n'.
</syntaxhighlight>
|-
! <code>[^…]</code>
| Matches every character except the ones inside brackets.
| <syntaxhighlight lang="perl">
$string1 = "Hello World\n";
if ($string1 =~ m/[^abc]/) {
  print "$string1 contains a character other than ";
  print "a, b, and c.\n";
}
</syntaxhighlight>
'''Output:'''
<syntaxhighlight lang="output">
Hello World
 contains a character other than a, b, and c.
</syntaxhighlight>
|}