Validating user input (email-shaped, phone-shaped, postcode-shaped) and seeing exactly which inputs pass and fail. Parsing log lines, extracting fields, building log filters. Designing find-and-replace patterns before running them across a real codebase. Debugging a regex you copied from Stack Overflow that doesn't work — paste it here, see what it actually matches.

\b\w+@\w+\.\w+\b — email-ish ^\s*$ — empty/whitespace-only line (with m flag) (? \d{4})-(? \d{2}) — named capture groups (?:.*) — non-capturing group (?=foo) / (?!foo) — lookahead / negative lookahead

JavaScript ≠ PCRE. No \K , no recursive patterns, lookbehind only since ES2018. Patterns from Perl, PHP, or Python often need adjustment. Without g flag you get the first match only. Add g for "find all"; combine with m if anchors should match per-line. Greedy vs lazy. .* grabs as much as possible; .*? grabs as little. The difference between matching hi and there as one block vs two. Anchors at line vs string boundaries. ^ and $ match string ends by default; with m flag they match each line. Replacement specials. $& is the whole match; $1 , $2 , … are capture groups; $$ is a literal $ . Forgetting that is a common source of "why is my regex eating my dollars". Don't parse HTML with regex for anything serious. The classic warning is true: nested tags, comments, and CDATA need a real parser. Regex is fine for one-off log scraping or controlled inputs.

Probador de Regex

Prueba expresiones regulares de JavaScript en vivo. Coincidencias, grupos y reemplazos al instante.

Pattern

/ /

Flags: g global · i case-insensitive · m multi-line · s dotAll · u unicode · y sticky

Test string

Highlighted matches

Capture groups

Introduce un valor arriba para ver el resultado.

Replace

Use $1, $2 for groups · $& for the whole match · $$ for literal $

Introduce un valor arriba para ver el resultado.

What is this for?

Regular expressions are dense and unforgiving. The way to write one that actually works is iteratively — pattern, sample text, see what matches, adjust. This tool gives you that loop in your browser using the JavaScript engine's native RegExp, plus capture-group inspection and a replacement preview. Patterns and inputs never leave the page.

When to use it

Validating user input (email-shaped, phone-shaped, postcode-shaped) and seeing exactly which inputs pass and fail.
Parsing log lines, extracting fields, building log filters.
Designing find-and-replace patterns before running them across a real codebase.
Debugging a regex you copied from Stack Overflow that doesn't work — paste it here, see what it actually matches.

Common patterns

\b\w+@\w+\.\w+\b — email-ish
^\s*$ — empty/whitespace-only line (with m flag)
(?<year>\d{4})-(?<month>\d{2}) — named capture groups
(?:.*) — non-capturing group
(?=foo) / (?!foo) — lookahead / negative lookahead

Common gotchas

JavaScript ≠ PCRE. No \K, no recursive patterns, lookbehind only since ES2018. Patterns from Perl, PHP, or Python often need adjustment.
Without g flag you get the first match only. Add g for "find all"; combine with m if anchors should match per-line.
Greedy vs lazy. .* grabs as much as possible; .*? grabs as little. The difference between matching <b>hi</b> and <i>there</i> as one block vs two.
Anchors at line vs string boundaries. ^ and $ match string ends by default; with m flag they match each line.
Replacement specials. $& is the whole match; $1, $2, … are capture groups; $$ is a literal $. Forgetting that is a common source of "why is my regex eating my dollars".
Don't parse HTML with regex for anything serious. The classic warning is true: nested tags, comments, and CDATA need a real parser. Regex is fine for one-off log scraping or controlled inputs.