bichonfrise74's scratchpad

Regex Summary

Metacharacter	Meaning
[abc]	Any one of a, b, or c.
[abc]	Anything other than a, b, and c.
\d \D	A digit; a non-digit.
\w \W	A 'word' character; a non-'word' character.
\s \S	A whitespace character; a non-whitespace character.
\b	The boundary between a \w character and a \W character
.	Any single character (apart from a new line).
(abc)	The phrase 'abc' as a group.
?	Preceding character or group may be present 0 or 1 times.
+	Preceding character or group is present 1 or more times.
*	Preceding character or group may be present 0 or more times.
{x,y}	Preceding character or group is present between x and y times.
{,y}	Preceding character or group is present at most y times.
{x,}	Preceding character or group is present at least x times.
{x}	Preceding character or group is present x times.

Non-greediness For Quantifiers

Syntax: (pattern)+?

(pattern)*?

The metacharacters + and * are greedy by default and will try to match as much as possible of the referenced string (while still achieving a full pattern match). This 'greedy' behavior can be turned off by placing a ? immediately after the respective metacharacter. A non-greedy match finds the minimum number of characters matching the pattern.

Grouping and Alternation

| For Alternation

Syntax: pattern1|pattern2

By separating two patterns with |, we can specify that a match on one or the other should be attempted.

() For Grouping And Backreferences ('Capturing')

Syntax: (pattern)

This will group elements in pattern. If those elements are matched, a backreference is made to one of the numeric special variables ($1, $2, $3 etc.)

(?:) For Non-backreferenced Grouping ('Clustering')

Syntax: (?:pattern)

This will group elements in pattern without making backreferences.

Lookahead/behind Assertions

(?=) For Positive Lookahead

Syntax: pattern1(?=pattern2)

This lets us look for a match on 'pattern1 followed by pattern2', without backreferencing pattern2.

(?!) For Negative Lookahead

Syntax: pattern1(?!pattern2)

This lets us look for a match on 'pattern1 not followed by pattern2', without backreferencing pattern2.

(?<=) For Positive Lookbehind

Syntax: pattern1(?<=pattern2)

This lets us look for a match on 'pattern1 preceded by pattern2', without backreferencing pattern2. This only works if pattern2 is of fixed width.

(?<!) For Negative Lookbehind

Syntax: pattern1(?<!pattern2)

This lets us look for a match on 'pattern1 not preceded by pattern2', without backreferencing pattern2. This only works if pattern2 is of fixed width.

Backreference Variables

Variable	Description
\num (num = 1, 2, 3…)	Within a regular expression, \num returns the substring that was matched with the numth grouped pattern in that regexp.
$num (num = 1, 2, 3…)	Outside a regular expression, $num returns the substring that was matched with the numth grouped pattern in that regexp.
$+	This returns the substring matched with the last grouped pattern in a regexp.
$&	This returns the string that matched the whole regexp – this will include portions of the string that matched (?:) groups, which are otherwise not backreferenced.
$`	This returns everything preceding the matched string in $&.
$'	This returns everything following the matched string in $&.


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