Reference

Regular expressions, explained

A regular expression is a small program that matches text. These entries explain the constructs that matter in daily work, how engines disagree about them, and where a pattern can become slow or wrong.

What a regex actually does

A pattern describes a set of strings. The engine walks the input, tries the pattern at each position, and returns the first match unless a global flag asks for more. Quantifiers and alternation make that search backtrack: the engine can undo a choice and try another, which is both the source of the expressive power and of the worst performance failures.

Almost every syntax question is answered by knowing three things: which engine you are using, which flags are active, and whether the pattern is anchored. Entries below name the engine when behaviour differs.

Engines are not interchangeable

JavaScript follows ECMAScript, which supports lookbehind and named groups but not possessive quantifiers or atomic groups. PCRE and Java add those. Python's re requires fixed-width lookbehind. RE2 and similar linear-time engines drop backreferences and lookaround entirely to guarantee performance.

A pattern that works in one language can fail to compile in another, or compile and mean something slightly different. Portability starts with avoiding the exotic constructs unless they are needed.

All entries

Character classesSquare brackets match one character from a set, with ranges, negation and shorthand classes such as \d and \w. QuantifiersQuantifiers repeat the previous atom: * zero or more, + one or more, ? optional, and {n,m} an explicit count. Greedy and lazy matchingGreedy quantifiers take as much as possible; adding ? makes them lazy and take as little as possible. Anchors and word boundariesAnchors match positions, not characters: ^ and $ for line or string edges, \b for a word boundary. Capturing groupsParentheses group a subpattern and capture its text; groups are numbered by the order of their opening parenthesis. Non-capturing groups(?:...) groups a subpattern for quantifiers and alternation without creating a capture. Lookahead and lookbehindLookaround asserts what comes before or after a position without consuming characters. BackreferencesA backreference matches the same text captured earlier by a group, such as \1 or \k<name>. Regex flagsFlags change how a pattern is interpreted: global, case-insensitive, multiline, dot-all, Unicode, sticky and indices. Catastrophic backtrackingNested quantifiers can make a regex take exponential time, a failure mode known as ReDoS.

Use these with

Regex Tester

Test a JavaScript regular expression against your text, inspect groups and preview replacements.

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Base64 Encoder

Encode text to Base64 or decode it back, with UTF-8 and URL-safe support.

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