Lookbehind

Lookbehind is a zero-width assertion used in regular expressions that checks whether the characters immediately to the left of the current position match a specified pattern, without consuming those characters as part of the match.

There are positive lookbehinds, written as (?<=...), and negative lookbehinds, written as (?<!...). In both cases the assertion does not advance the search position; it only determines whether a match can occur at that point. Many regex engines require the lookbehind to have a fixed length, but some modern engines support variable-length lookbehinds under certain constraints, often with performance considerations.

Examples help illustrate the concept. For instance, in the string "foo123" the pattern (?<=foo)\d+ matches the digits "123" because they are immediately preceded by "foo". Conversely, the pattern (?<!foo)\d+ would also match "123" in "bar123" but not in "foo123" since the latter is preceded by "foo". Lookbehinds can be combined with other constructs, such as lookaheads, to form more precise patterns.

Lookbehind is commonly used to enforce contextual boundaries without including the preceding text in the match, extract data that follows a specific label, or conditionally match content based on what appears to on its left. It is supported in most modern regex engines, though its availability and fixed-length requirement vary by language and implementation.

Limitations and alternatives: if a target engine does not support lookbehind, similar results may be achieved with capturing groups and lookaheads, or by restructuring the pattern to avoid the need for left-context assertions.