# Regular Expressions/Perl-Compatible Regular Expressions

Perl has a richer and more predictable syntax than even the POSIX Extended Regular Expressions syntax. An example of its predictability is that \ always quotes a non-alphanumeric character. An example of something that is possible to specify with Perl but not POSIX is whether part of the match wanted to be greedy or not. For instance in the pattern /a.*b/, the .* will match as much as it can, while in the pattern /a.*?b/, .*? will match as little. So given the string "a bad dab", the first pattern will match the whole string, and the second will only match "a b".
For these reasons, many other utilities and applications have adopted syntaxes that look a lot like Perl's. For example, Java, Ruby, Python, PHP, exim, BBEdit, and even Microsoft's .NET Framework all use regular expression syntax similar to that used in perl. Not all "Perl-compatible" regular expression implementations are identical, and many implement only a subset of Perl's features.

## Examples

Conventions used in the examples: 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 collision". See 'perldoc perlre' for more details.
In the table heading below, "M-c" stands for "Metacharacters".
| M-c | Description | Example All the if statements return a TRUE value. |
| . | Normally matches any character except a newline. Within square brackets the dot is literal. | if ("Hello World\n" =~ m/...../) { print "Yep"; # Has length >= 5\n"; } |
| ( ) | 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. | if ("Hello World\n" =~ m/(H..).(o..)/) { print "We matched '$1' and '$2'\n"; } Output:We matched 'Hel' and 'o W'; |
| + | Matches the preceding pattern element one or more times. | if ("Hello World\n" =~ m/l+/) { print "One or more \"l\"'s in the string\n"; } |
| ? | Matches the preceding pattern element zero or one times. | if ("Hello World\n" =~ m/H.?e/) { print "There is an 'H' and a 'e' separated by "; print "0-1 characters (Ex: He Hoe)\n"; } |
| ? | Modifies the *, +, or {M,N}'d regexp that comes before to match as few times as possible. | if ("Hello World\n" =~ m/(l.+?o)/) { print "Yep"; # The non-greedy match with 'l' followed # by one or more characters is 'llo' rather than 'llo wo'. } |
| * | Matches the preceding pattern element zero or more times. | if ("Hello World\n" =~ m/el*o/) { print "There is an 'e' followed by zero to many "; print "'l' followed by 'o' (eo, elo, ello, elllo)\n"; } |
| {M,N} | Denotes the minimum M and the maximum N match count. | if ("Hello World\n" =~ m/l{1,2}/) { print "There is a substring with at least 1 "; print "and at most 2 l's in the string\n"; } |
| [...] | Denotes a set of possible character matches. | if ("Hello World\n" =~ m/[aeiou]+/) { print "Yep"; # Contains one or more vowels } |
| | | Separates alternate possibilities. | if ("Hello World\n" =~ m/(Hello|Hi|Pogo)/) { print "At least one of Hello, Hi, or Pogo is "; print "contained in the string.\n"; } |
| \b | Matches a word boundary. | if ("Hello World\n" =~ m/llo\b/) { print "There is a word that ends with 'llo'\n"; } |
| \w | Matches an alphanumeric character, including "_". | if ("Hello World\n" =~ m/\w/) { print "There is at least one alphanumeric "; print "character in the string (A-Z, a-z, 0-9, _)\n"; } |
| \W | Matches a non-alphanumeric character, excluding "_". | if ("Hello World\n" =~ m/\W/) { print "The space between Hello and "; print "World is not alphanumeric\n"; } |
| \s | Matches a whitespace character (space, tab, newline, form feed) | if ("Hello World\n" =~ m/\s.*\s/) { print "There are TWO whitespace characters, which may"; print " be separated by other characters, in the string."; } |
| \S | Matches anything BUT a whitespace. | if ("Hello World\n" =~ m/\S.*\S/) { print "Contains two non-whitespace characters " . "separated by zero or more characters."; } |
| \d | Matches a digit, same as [0-9]. | if ("99 bottles of beer on the wall." =~ m/(\d+)/) { print "$1 is the first number in the string'\n"; } |
| \D | Matches a non-digit. | if ("Hello World\n" =~ m/\D/) { print "There is at least one character in the string"; print " that is not a digit.\n"; } |
| ^ | Matches the beginning of a line or string. | if ("Hello World\n" =~ m/^He/) { print "Starts with the characters 'He'\n"; } |
| $ | Matches the end of a line or string. | if ("Hello World\n" =~ m/rld$/) { print "Is a line or string "; print "that ends with 'rld'\n"; } |
| \A | Matches the beginning of a string (but not an internal line). | if ("Hello\nWorld\n" =~ m/\AH/) { print "Yep"; # The string starts with 'H'. } |
| \Z | Matches the end of a string (but not an internal line). | if ("Hello\nWorld\n"; =~ m/d\n\Z/) { print "Yep"; # Ends with 'd\\n'\n"; } |
| [^...] | Matches every character except the ones inside brackets. | if ("Hello World\n" =~ m/[^abc]/) { print "Yep"; # Contains a character other than a, b, and c. } |

## Use in Tools

Tools and languages that utilize Perl regular expression syntax include:
* Java
* Leafnode
* Perl
* Python
* PHP

## See also

* Perl Programming/Regular Expressions Reference

## Links

* Perl regular expressions at perl.org
* Perl Compatible Regular Expressions library at pcre.org
* Perl Regular Expression Syntax at boost.org
* W:Regular_expressions#Perl