Matching

Syntax of the builtin regular expression library

In the event that an RE could match more than one substring of a given string, the RE matches the one starting earliest in the string. If the RE could match more than one substring starting at that point, its choice is determined by its preference: either the longest substring, or the shortest.

Most atoms, and all constraints, have no preference. A parenthesized RE has the same preference (possibly none) as the RE. A quantified atom with quantifier {m} or {m}? has the same preference (possibly none) as the atom itself. A quantified atom with other normal quantifiers (including {m,n} with m equal to n) prefers longest match. A quantified atom with other non-greedy quantifiers (including {m,n}? with m equal to n) prefers shortest match. A branch has the same preference as the first quantified atom in it which has a preference. An RE consisting of two or more branches connected by the $\vert$ operator prefers longest match.

Subject to the constraints imposed by the rules for matching the whole RE, subexpressions also match the longest or shortest possible substrings, based on their preferences, with subexpressions starting earlier in the RE taking priority over ones starting later. Note that outer subexpressions thus take priority over their component subexpressions.

Note that the quantifiers {1,1} and {1,1}? can be used to force longest and shortest preference, respectively, on a subexpression or a whole RE.

Match lengths are measured in characters, not collating elements. An empty string is considered longer than no match at all. For example, bb* matches the three middle characters of `abbbc', (week$\vert$wee)(night$\vert$knights) matches all ten characters of `weeknights', when (.*).* is matched against abc the parenthesized subexpression matches all three characters, and when (a*)* is matched against bc both the whole RE and the parenthesized subexpression match an empty string.

If case-independent matching is specified, the effect is much as if all case distinctions had vanished from the alphabet. When an alphabetic that exists in multiple cases appears as an ordinary character outside a bracket expression, it is effectively transformed into a bracket expression containing both cases, so that x becomes `$[xX]$'. When it appears inside a bracket expression, all case counterparts of it are added to the bracket expression, so that $[x]$ becomes $[xX]$ and $[^x]$ becomes `$[^xX]$'.

If newline-sensitive matching is specified, . and bracket expressions using ^ will never match the newline character (so that matches will never cross newlines unless the RE explicitly arranges it) and ^ and $ will match the empty string after and before a newline respectively, in addition to matching at beginning and end of string respectively. ARE $\backslash$A and $\backslash$Z continue to match beginning or end of string only.

If partial newline-sensitive matching is specified, this affects . and bracket expressions as with newline-sensitive matching, but not ^ and `$'.

If inverse partial newline-sensitive matching is specified, this affects ^ and $ as with newline-sensitive matching, but not . and bracket expressions. This isn't very useful but is provided for symmetry.