Regex tester

How it works

Your browser builds a RegExp from the pattern string and selected flags (g, i, m, s). Matches are found with the same semantics as JavaScript itself — useful for Node, Deno, and browser code. The highlight layer mirrors your text with <mark> around each match; the textarea sits on top with transparent text so you still edit normally.

Without the g flag, only the first match is found for iteration, but you can still test anchoring and capture groups. With g, all non-overlapping matches are listed in order. Empty matches advance the search index so the loop cannot hang on patterns like (?=a).

When the pattern is invalid, the page shows the engine’s error message in red and clears highlights so you are never left guessing. Live mode debounces keystrokes so typing stays responsive on long logs.

Common use cases

• Debug log parsing — prove a pattern extracts order IDs, timestamps, or severity levels before you commit it to a parser.
• Validate form and API rules — test email-ish patterns, phone formats, or slug rules the same way String.prototype.match will behave in production.
• Capture group rehearsal — confirm $1, $2 line up when refactoring a split or replace pipeline.
• Flags and multiline behavior — flip m and s to see how ^, $, and . change across newlines.
• Teach or document patterns — share a readable screen capture or walk through a pattern in a PR without shipping data to a third party.
• Compare against other engines — when behavior must match JavaScript specifically, avoid surprises from PCRE-only online testers.

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