Regex Tester & Explainer

Test a regular expression against sample text and get every match, capture group, named group, and position — plus a plain-English explanation of the pattern. Built for agents and developers writing extraction rules.

Live on the Apify Store — run it instantly or call it as an agent tool via Apify MCP.

What you get

• Every match with its index (position) in the text
• Capture groups (positional) and named groups per match
• Plain-English explanation — a token-by-token breakdown of what the pattern does
• ReDoS safety — catastrophic-backtracking patterns are flagged, and matching is time-boxed and length-capped so a run never hangs
• Works for a single test per run; safe defaults, sane caps

Input

{
  "pattern": "(?<user>\\w+)@(?<domain>[\\w.]+)",
  "text": "Contact alice@example.com or bob@test.org for details.",
  "flags": "g",
  "explain": true
}

• pattern (string, required) — the regex, no slashes
• text (string, required) — text to test against
• flags (string, default "g") — any of g i m s u y d (g is always applied)
• explain (boolean, default true) — include the plain-English explanation

Output

{
  "pattern": "(?<user>\\w+)@(?<domain>[\\w.]+)",
  "flags": "g",
  "matchCount": 2,
  "matches": [
    {
      "match": "alice@example.com",
      "index": 8,
      "groups": ["alice", "example.com"],
      "namedGroups": { "user": "alice", "domain": "example.com" }
    },
    {
      "match": "bob@test.org",
      "index": 29,
      "groups": ["bob", "test.org"],
      "namedGroups": { "user": "bob", "domain": "test.org" }
    }
  ],
  "explanation": "Pattern /(?<user>\\w+)@(?<domain>[\\w.]+)/g\nFlags: global (find all matches)\nSteps:\n1. Begin a named group \"user\"\n2. Match a word character (a-z, A-Z, 0-9, _) (one or more times)\n3. End group\n4. Match the literal text \"@\"\n5. Begin a named group \"domain\"\n6. Match any character in the set [\\w.] (one or more times)\n7. End group",
  "warnings": []
}

Pricing

Pay-per-event: one charge per test run.