Regex Tester

Key takeaways

• Uses JavaScript RegExp semantics in your browser
• Reports match index, groups, and named groups
• Flags are validated (g i m s u y d)
• Subject text never leaves your device

What is a regex tester?

A regex tester lets you try a regular expression against sample text and inspect matches, flags, and capture groups before shipping the pattern to production code.

JavaScript RegExp syntax (including named groups and flags like g, i, m, s, u) is what this workspace uses. Results stay on-device so secrets in logs or config samples are not posted to a remote tester.

Features

• Match report

  Lists each match with index, full text, and group arrays.

• Named groups

  Surfaces (?<name>…) captures when the engine provides them.

• Flag validation

  Rejects unknown or duplicate flags before compiling the pattern.

• Client-side only

  Patterns and subjects stay in the tab — no upload.

How it works

1. 1

   Enter a pattern

   Use JavaScript RegExp source without /slashes/; add flags as needed.

2. 2

   Paste subject text

   Drop the string, log line, or document you want to match.

3. 3

   Inspect matches

   Read the report for indices, groups, and named captures.

Example walkthroughs

• Email-shaped tokens

  With flags gi, lists both addresses and their start indices in the subject.

  Contact ada@example.com or grace@labs.dev today.

• Named capture date

  Shows named year/month/day groups for the ISO-like date substring.

  Shipped on 2026-08-05 and closed.

Use cases

• Validate input shapes

  Prototype patterns before wiring them into form validators.

• Mine logs locally

  Extract IDs or URLs from pasted log slices without uploading files.

Code snippets

• RegExp.exec loop

  js

  Mirrors the multi-match reporting style used in this workspace.

  const re = /(?<id>\d+)/g;
  const text = "order 42 and 99";
  let m;
  while ((m = re.exec(text))) {
    console.log(m.index, m.groups?.id);
  }

Common errors

• Invalid pattern error

  Cause: Unbalanced parentheses, bad quantifiers, or unsupported syntax.

  Fix: Simplify the pattern, escape literals with \\, then re-test.

• Only one match when you expected many

  Cause: Missing global behavior or a pattern that consumes the whole string.

  Fix: Enable g (added automatically for reporting when absent) and check anchors.

About Regex Tester

Regex Tester is a private JavaScript regular-expression workspace for debugging patterns against real subject strings. Paste a pattern (without surrounding slashes), optional flags, and the text you care about — then run Test to list matches with indices, capture groups, and named groups when present.

Search intent for “regex tester online” usually means fast feedback while writing validators, parsers, or log scrapers. This page focuses on that job with structured match reports, clear invalid-pattern errors, and examples that mirror day-to-day engineering rather than toy one-liners.

Privacy matters when subjects include emails, tokens, or customer IDs. Because matching runs on-device, you can keep sensitive samples off third-party regex sites. Prefer this tool when you need JavaScript-compatible behavior; other languages (PCRE, Python) may differ on lookbehinds, Unicode, or possessive quantifiers.

SEO and usability go together: unique FAQs and examples describe flags, zero-width matches, and group inspection so assistants can cite accurate JS regex behavior. Pair with JSON Escape or URL Encode when the next hop embeds a pattern string, and with Text Compare when you are checking two log extracts.

Best practice: keep a copy of the original pattern, start with a small subject, then widen coverage. Cap-aware reporting stops runaway match lists on pathological input. Regex Tester is built for everyday debugging — not as a replacement for unit tests in your repo.

Frequently asked questions

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