Regex Tester API

Run a JavaScript regular expression against text: every match with its position and capture groups, an optional replacement result, and a plain-English explanation of the pattern. Runs are time-limited.

POST /api/v1/tools/regex-testerMCP regex_testerDeveloper Tools

Test JavaScript regular expressions live: highlight every match, inspect numbered and named capture groups, toggle flags and preview replacements in your browser.

Input

Send these as a JSON object. Unknown fields are rejected.

pattern: The pattern without slashes, e.g. (\d{4})-(\d{2})-(\d{2}). JavaScript (ECMAScript) syntax. (string, up to 2000 characters, required)
flags: Flags from "dgimsuvy", each at most once; g lists every match, without g only the first. (string, up to 8 characters, default "g")
text: The text to search (up to 50,000 characters). (string, up to 50000 characters, default "")
replacement: Optional replacement string ($1, $<name>, $& supported). When given, the replaced text is returned as replaced. (string, up to 10000 characters)
maxMatches: Most matches to return (1 to 1000). (integer, 1 to 1000, default 100)
explain: Include a plain-English explanation of each part of the pattern. (boolean, default true)

Try it in the browser JSON Schema Get an API key

curl https://pithlab.com/api/v1/tools/regex-tester \n  -H "Authorization: Bearer $PITHLAB_KEY" \n  -H "Content-Type: application/json" \n  -d '{"pattern":"(?<year>\\d{4})-(?<month>\\d{2})","flags":"g","text":"Released 2024-03, updated 2025-11.","replacement":"$<month>/$<year>"}'

Example response

{
  "ok": true,
  "result": {
    "matchCount": 2,
    "truncated": false,
    "matches": [
      {
        "index": 9,
        "end": 16,
        "text": "2024-03",
        "groups": [
          "2024",
          "03"
        ],
        "named": {
          "year": "2024",
          "month": "03"
        }
      },
      {
        "index": 26,
        "end": 33,
        "text": "2025-11",
        "groups": [
          "2025",
          "11"
        ],
        "named": {
          "year": "2025",
          "month": "11"
        }
      }
    ],
    "replaced": "Released 03/2024, updated 11/2025."
  }
}