Regex Explainer
Paste a regular expression and read it in plain English, part by part: groups, classes, quantifiers, anchors, lookarounds, flags — hazards flagged.
A line-by-line explanation of the pattern from its grammar (no AI), the group count and names, what each flag does, and warnings such as nested unbounded repeats or a greedy ".*".
Example: ^(?<user>[\w.+-]+)@([a-z0-9-]+\.)+[a-z]{2,}$ reads as: start; named group "user" of word characters, dots, plus or minus; "@"; one or more of (letters, digits or hyphens then a dot); 2+ letters; end.
Read the pattern
the way the engine does.
How the pattern is parsed, where the words come from, and which hazards are pointed out.
Parsing
The pattern is parsed with a small recursive-descent parser for the ECMAScript grammar: alternation, sequences, groups (capturing, named, non-capturing, lookahead, lookbehind), character classes with ranges and negation, escapes (\d \w \s \b, hex and unicode, control, back-references), anchors, the dot, and quantifiers with lazy variants. It is first compiled with the browser's own RegExp so a genuinely invalid pattern reports the engine's error, not a guess.
The explanations
Every sentence is attached to a grammar rule — "one character from the set", "zero or more times, as many as possible" — and assembled from the tree, with a quantified group shown on one line and its contents indented beneath. There is no language model involved; the same pattern always gives the same words, and a construct the parser does not know is reported rather than paraphrased.
Hazards
Two warnings are about performance: a repeated group that itself contains an unbounded repeat ((a+)+) can backtrack exponentially on non-matching input, and a greedy ".*" grabs as much as possible before backtracking. Two are about intent: a pattern without ^ … $ matches anywhere, and a bare | matches the empty string. They are hints, not verdicts. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
Last reviewed 20 September 2026. How results are checked: How we verify.