Regex Explainer

How to use the regex explainer

1. Paste the regular expression.
2. Add its flags if it has any.
3. Read the explanation piece by piece.

About this tool

Inheriting a regular expression someone else wrote is a common and unpleasant experience. This breaks one into its pieces and says what each does, in order, including the flags — which are easy to overlook and change the meaning of the whole pattern.

Distinctions that matter are called out specifically: greedy versus lazy quantifiers, capturing versus non-capturing groups, and lookahead versus lookbehind. Those four account for most of the confusion when a pattern nearly works.

Be clear about what this is. It explains each piece in the order it appears — a reading aid, not a parser. It will not tell you that two parts can never both match, or summarise the expression as a whole. It answers what each symbol is doing, which is usually the actual question.

Common uses

• Understanding a pattern found in an existing codebase.
• Learning regex syntax from real examples.
• Checking a pattern does what its author claimed.

Frequently asked questions

What is the difference between greedy and lazy?

A greedy quantifier takes as much as it can and gives back only if it must. A lazy one, written with a trailing ?, takes as little as possible. It is the usual reason .* swallows too much.

What does (?: ) mean?

A group that is not captured. It groups the pattern for a quantifier or alternation without producing a numbered result.

Does this validate my pattern?

It reports a syntax error if the pattern will not compile, and warns about the nested-quantifier shape that risks catastrophic backtracking. It does not check whether the pattern matches what you intended.