LastIndex

Lastindex is a term used in string processing and regular expressions to refer to the position of a relevant match within a string, or to the index used to resume a search when performing repeated pattern matching. The exact meaning and naming vary by language and library.

In JavaScript, the lastIndex property belongs to RegExp objects. It indicates the position at which the next search should start when the regular expression has the global or sticky flag. After a successful exec call, lastIndex is updated to the index after the end of the matched substring; if no match occurs, lastIndex is reset to 0.

In Python, the Match object exposes lastindex, a numeric attribute that gives the index (1-based) of the last captured group, or None if there are no capturing groups or the last group did not participate in the match. This helps identify which capturing group was last used in a match result.

Other contexts may use similar concepts under different names, such as lastIndex or lastindex, with varying conventions for 0-based vs 1-based indexing and for how zero-length matches are handled. The precise behavior depends on the language, library, and whether the operation is operating in a global search mode or a single-shot match.

Practical use of lastindex-related concepts includes iterative searches across a string, retrieving information from capturing groups, and resuming pattern scans after partial matches. Common pitfalls involve off-by-one errors, differing index bases between languages, and handling cases where no match or no capturing groups are present.