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A [[regular expression]] ( also "RegEx" or "regex" ) is a string that is used to describe or match a set of strings according to certain [[syntax]] rules. 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|version]]s.

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Despite this variability, and because regular expressions can be difficult to both explain and understand without examples, this article provides a basic description of some of the properties of regular expressions by way of illustration.

== Conventions ==

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, m/[^abc]/ could also be rendered as /[^abc]/. 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#Delimiter collision|delimiter collision]]". See '[http://perldoc.perl.org/perlre.html perldoc perlre]' 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

Also worth noting is that these regular expressions are all Perl-like syntax. Standard POSIX regular expressions are different.

== Examples ==

Unless otherwise indicated, the following examples conform to the [[Perl]] programming language, release 5.8.8, [[January 31]] [[2006]]. The syntax and conventions used in these examples coincide with that of other programming environments as well (e.g., see Java in a Nutshell - Page 213, Python Scripting for Computational Science - Page 320, Programming PHP - Page 106 ).

<table class="wikitable">

<tr>

<th>Metacharacter(s)</th>

<th>Description</th>

<th>Example

<br>Note that all the if statements return a TRUE value</th>

</tr>

<tr>

<td>'''.'''</td>

<td>Normally matches any character except a newline. Within square brackets the dot is literal.</td>

<td align="left">

<pre>

$string1 = "Hello World\n";

if ($string1 =~ m/...../) {

print "$string1 has length >= 5\n";

}

</pre></td>

</tr>

<tr>

<td>( )</td>

<td>Groups a series of pattern elements to a single element. When you match a pattern within parentheses, you can use any of $1, $2, ... later to refer to the previously matched pattern.</td>

<td align="left">

<pre>

$string1 = "Hello World\n";

if ($string1 =~ m/(H..).(o..)/) {

print "We matched '$1' and '$2'\n";

}

</pre>'''Output:'''<pre>

We matched 'Hel' and 'o W';

</pre></td>

</tr>

<tr>

<td>+</td>

<td>Matches the preceding pattern element one or more times.</td>

<td align="left">

<pre>

$string1 = "Hello World\n";

if ($string1 =~ m/l+/) {

print "There are one or more consecutive letter \"l\"'s in $string1\n";

}

</pre>'''Output:'''<pre>

There are one or more consecutive letter "l"'s in Hello World

</pre></td>

</tr>

<tr>

<td>?</td>

<td>Matches the preceding pattern element zero or one times.</td>

<td align="left">

<pre>

$string1 = "Hello World\n";

if ($string1 =~ m/H.?e/) {

print "There is an 'H' and a 'e' separated by ";

print "0-1 characters (Ex: He Hoe)\n";

}

</pre></td>

</tr>

<tr>

<td>?</td>

<td>Modifies the *, +, or {M,N}'d regexp that comes before

to match as few times as possible.</td>

<td align="left">

<pre>

$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";

}

</pre></td>

</tr>

<tr>

<td>*</td>

<td>Matches the preceding pattern element zero or more times.</td>

<td align="left">

<pre>

$string1 = "Hello World\n";

if ($string1 =~ m/el*o/) {

print "There is an 'e' followed by zero to many ";

print "'l' followed by 'o' (eo, elo, ello, elllo)\n";

}

</pre></td>

</tr>

<tr>

<td>{M,N}</td>

<td>Denotes the minimum M and the maximum N match count.</td>

<td align="left">

<pre>

$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";

}

</pre>

</td>

</tr>

<tr>

<td>[...]</td>

<TD>Denotes a set of possible character matches.</TD>

<td align="left">

<pre>

$string1 = "Hello World\n";

if ($string1 =~ m/[aeiou]+/) {

print "$string1 contains one or more vowels.\n";

}

</pre></td>

</tr>

<tr>

<td>|</td>

<td>Separates alternate possibilities.</td>

<td align="left">

<pre>

$string1 = "Hello World\n";

if ($string1 =~ m/(Hello|Hi|Pogo)/) {

print "At least one of Hello, Hi, or Pogo is ";

print "contained in $string1.\n";

}

</pre></td>

</tr>

<tr>

<td>\b</td>

<td>Matches a word boundary.</td>

<td align="left">

<pre>

$string1 = "Hello World\n";

if ($string1 =~ m/llo\b/) {

print "There is a word that ends with 'llo'\n";

} else {

print "There are no words that end with 'llo'\n";

}

</pre></td>

</tr>

<tr>

<td>\w</td>

<td>Matches an alphanumeric character, including "_".</td>

<td align="left">

<pre>

$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";

}

</pre></td>

</tr>

<tr>

<td>\W</td>

<td>Matches a non-alphanumeric character, excluding "_".</td>

<td align="left">

<pre>

$string1 = "Hello World\n";

if ($string1 =~ m/\W/) {

print "The space between Hello and ";

print "World is not alphanumeric\n";

}

</pre></td>

</tr>

<tr>

<td>\s</td>

<td>Matches a whitespace character (space, tab, newline, form feed)</td>

<td align="left">

<pre>

$string1 = "Hello World\n";

if ($string1 =~ m/\s.*\s/) {

print "There are TWO whitespace characters, which may";

print " be separated by other characters, in $string1";

}

</pre></td>

</tr>

<tr>

<td>\S</td>

<td>Matches anything BUT a whitespace.</td>

<td align="left">

<pre>

$string1 = "Hello World\n";

if ($string1 =~ m/\S.*\S/) {

print "There are TWO non-whitespace characters, which";

print " may be separated by other characters, in $string1";

}

</pre></td>

</tr>

<tr>

<td>\d</td>

<td>Matches a digit, same as [0-9].</td>

<td align="left">

<pre>

$string1 = "99 bottles of beer on the wall.";

if ($string1 =~ m/(\d+)/) {

print "$1 is the first number in '$string1'\n";

}

</pre>'''Output:'''<pre>

99 is the first number in '99 bottles of beer on the wall.'

</pre></td>

</tr>

<tr>

<td>\D</td>

<td>Matches a non-digit.</td>

<td align="left">

<pre>

$string1 = "Hello World\n";

if ($string1 =~ m/\D/) {

print "There is at least one character in $string1";

print " that is not a digit.\n";

}

</pre></td>

</tr>

<tr>

<td>^</td>

<td>Matches the beginning of a line or string.</td>

<td align="left">

<pre>

$string1 = "Hello World\n";

if ($string1 =~ m/^He/) {

print "$string1 starts with the characters 'He'\n";

}

</pre></td>

</tr>

<tr>

<td>$</td>

<td>Matches the end of a line or string.</td>

<td align="left">

<pre>

$string1 = "Hello World\n";

if ($string1 =~ m/rld$/) {

print "$string1 is a line or string ";

print "that ends with 'rld'\n";

}

</pre></td>

</tr>

<tr>

<td>\A</td>

<td>Matches the beginning of a string (but not an internal line).</td>

<td align="left">

<pre>

$string1 = "Hello\nWorld\n";

if ($string1 =~ m/\AH/) {

print "$string1 is a string ";

print "that starts with 'H'\n";

}

</pre></td>

</tr>

<tr>

<td>\Z</td>

<td>Matches the end of a string (but not an internal line).</td>

<td align="left">

<pre>

$string1 = "Hello\nWorld\n";

if ($string1 =~ m/d\n\Z/) {

print "$string1 is a string ";

print "that ends with 'd\\n'\n";

}

</pre></td>

</tr>

<tr>

<td>[^...]</td>

<td>Matches every character except the ones inside brackets.</td>

<td align="left">

<pre>

$string1 = "Hello World\n";

if ($string1 =~ m/[^abc]/) {

print "$string1 contains a character other than ";

print "a, b, and c\n";

}

</pre></td>

</tr>

</table></center>

== Notes ==

{{Reflist}}

== See also ==

* [[Comparison of programming languages]]

[[Category:Perl]]

[[Category:Pattern matching]]

[[Category:Articles with example code]]

[[Category:Programming language topics]]