Regular Expressions

Here we summarize the syntax of Perl5 regular expressions, all of which are supported by the WebL. However, for a definitive reference, you should consult the perlre man page that accompanies the Perl5 distribution and also the book Programming Perl, 2nd Edition from O'Reilly & Associates. We need to point out here that for efficiency reasons the character set operator [...] is limited to work on only ASCII characters (Unicode characters 0 through 255). Other than this restriction, all Unicode characters should be useable in the package's regular expressions.

Regular expression within parentheses, character classes (e.g., [abcd]), ranges (e.g. [a-z]), and the patterns in See Atoms. Special backslashed characters work within a character class (except for backreferences and boundaries). \b is backspace inside a character class. Any other backslashed character matches itself. Expressions within parentheses are matched as subpattern groups and saved for use by certain methods.

Quantified Atoms
Pattern	Description
{n,m}	Match at least n but not more than m times.
{n,}	Match at least n times.
{n}	Match exactly n times.
*	Match 0 or more times.
+	Match 1 or more times.
?	Match 0 or 1 times.

By default, a quantified subpattern is greedy. In other words, it matches as many times as possible without causing the rest of the pattern not to match. To change the quantifiers to match the minimum number of times possible, without causing the rest of the pattern not to match, you may use a "?" right after the quantifier (See Quantified Atoms with Minimal Matching). Perl5 extended regular expressions are fully supported (See See Perl5 Extended Regular Expressions).

Regular Expression Tips

Combining regular expresions and WebL code might sometimes be a little confusing. The following tips might help:

When possible, write WebL regular expressions in single-back quotes e.g. `ab\nc`. This will switch off escape character expansion, and prevent WebL from complaining about illegal escape sequences like "\d".

When matching URLs, keep in mind that "." and "?" do not have a literal meaning in regular expressions. Use the "[]" character classes to match these symbols, e.g. write "www[.]xyz[.]com" instead of "www.xyz.com".

Pattern	Description
{n,m}?	Matches at least n but not more than m times.
{n,}?	Matches at least n times.
{n}?	Matches exactly n times.
*?	Matches 0 or more times.
+?	Matches 1 or more times.
??	Matches 0 or 1 times.

Pattern	Description
.	Matches everything except \n.
^	Null token matching the beginning of a string or line (i.e. the position right after a newline or right before the beginning of a string).
$	Null token matching the end of a string or line (i.e. the position right before a newline or right after the end of a string).
\b	Null token matching a word boundary (\w on one side and \W on the other).
\B	Null token matching a boundary that is not a word boundary.
\A	Matches only at beginning of string.
\Z	Matches only at end of string (or before newline at the end).
\n	Newline.
\r	Carriage return.
\t	Tab.
\f	Formfeed.
\d	Digit [0-9].
\D	Non-digit [^0-9].
\w	Word character [0-9a-zA-Z].
\W	Non-word character [^0-9a-zA-Z].
\s	A whitespace character [ \t\n\r\f].
\S	A non-whitespace character [^ \t\n\r\f].
\xnn	Hexadecimal representation of character.
\cD	Matches the corresponding control character.
\nn or \nnn	Octal representation of character unless a backreference.
\1, \2, \3, etc.	Matches whatever the first, second, third, etc. parenthesized group matched. This is called a backreference. If there is no corresponding group, the number is interpreted as an octal representation of a character.
\0	Matches null character.

Extended Pattern	Description
(?#text)	An embedded comment causing text to be ignored.
(?:regexp)	Groups things like "()" but does not cause the group match to be saved.
(?=regexp)	A zero-width positive lookahead assertion. For example, \w+(?=\s) matches a word followed by whitespace, without including whitespace in the match result.
(?!regexp)	A zero-width negative lookahead assertion. For example foo(?!bar) matches any occurrence of "foo" that is not followed by "bar". Remember that this is a zero-width assertion, which means that a(?!b)d will match ad because a is followed by a character that is not b (the d) and a d follows the zero-width assertion.
(?imsx)	One or more embedded pattern-match modifiers. i enables case insensitivity, m enables multiline treatment of the input, s enables single line treatment of the input, and x enables extended whitespace comments.