Regex Tester — Test & Debug Regular Expressions

The tester wraps three JavaScript primitives: new RegExp(pattern, flags) for compilation, String.prototype.matchAll for iteration, and String.prototype.replace for the rewrite. The whole thing is a pure function — no server call, no shared state, no dependency beyond the standard library.

1. Compile.Your pattern and the flag toggles are passed to the JavaScript engine's own RegExp constructor. The gflag is added unconditionally so iteration over multiple matches is well defined; any syntax error surfaces under the field with the engine's native diagnostic message rather than a generic "invalid pattern" string.
2. Match. The tool calls text.matchAll(regex) to walk every non-overlapping match. Zero-width matches (anchors like ^, lookarounds) are advanced past by the engine's AdvanceStringIndex algorithm in ECMA-262 §22.2.7.4, so a pattern like (?=\d) terminates instead of spinning forever.
3. Extract groups. For each match the tool reads m[1] through m[n] for numbered captures and m.groupsfor named captures. The numbered groups list also surfaces the name (if any) by parsing the source pattern's group order — that gives a clean $1 (year) label even when several named groups share a value.
4. Replace. A fresh compiled regex is passed to text.replace with the replacement string verbatim, so $1, $&, $`, $', and $<name> expand per ECMA-262 §22.1.3.18. The result is what your code would produce with the same pattern, byte for byte.
5. Cross-check. A second counter runs through replace with a counting callback. The Verified badge stays green only when both counters agree on the number of matches — the invariant countByMatchAll === countByReplace must hold for every well-formed input.
6. Explain.The Explain tab tokenises your pattern and names each construct — anchors, character classes, capture groups, quantifiers, escapes. It is a structural walk, not a full parser; tokens it cannot classify fall through as "literal", and the engine itself remains the authoritative source of truth for what compiles.

The flag set is the standard ECMA-262 alphabet: g global, i case-insensitive, m multiline, s dot-all, u Unicode, and y sticky. The Unicode flag enables \p{...} property escapes from Unicode TR-18, so a pattern like \p{L}+ matches a run of letters in any script — Sinhala (සිංහල), Tamil (தமிழ்), Cyrillic, accented Latin, CJK.

Performance: a megabyte of text against a typical pattern resolves in single-digit milliseconds on a mid-range phone. The page caps the input at 1,000,000 characters and the pattern at 10,000 — past those, the input is trimmed and a notice appears. Catastrophic backtracking is the one workload the tool cannot magically fix — if a pattern feels stuck, anchor it or rewrite the alternation; the iteration cap stops it from freezing the page.

• ECMA-262 — RegExp objects, flags, and AdvanceStringIndex
• ECMA-262 — String.prototype.matchAll iterator semantics
• ECMA-262 — String.prototype.replace and replacement-string tokens
• Unicode Technical Report #18 — Unicode Regular Expressions
• MDN — RegExp reference (flags, Unicode property escapes)
• MDN — Regular expressions cheatsheet

The tool's match and replace semantics were last cross-checked against the JavaScript engine on 2026-05-11. The page is reviewed whenever a relevant TC39 proposal lands — for example the v flag for set operations — or a Unicode TR update changes the property-escape definitions.