RegEx Tester API

Test and debug regular expressions live. Match highlighting, capture group inspection, flag toggles, and pattern explanations — 100% client-side.

The RegEx Tester is also available as a free REST API. Send a POST request with a JSON body to https://dot.tools/api/tools/regex-tester — no authentication, no API key, CORS enabled. Successful calls return { "ok": true, "result": { … } }; failures return HTTP 400 or 422 with an error body.

Request body

Field	Type	Required	Description
`pattern`	string	required	The JavaScript regular expression pattern (without slashes). Capped at 500 characters as a ReDoS guard.
`text`	string	required	The test text to match against. Capped at 100,000 characters as a ReDoS guard.
`flags`	object	required	Regular expression flags.

Response

Field	Type	Required	Description
`valid`	boolean	required	Whether the pattern compiled successfully.
`error`	unknown	required	Parse error message when the pattern is invalid.
`matchCount`	number	required	Total number of matches found.
`truncated`	boolean	required	Whether the matches list was capped at 1000 entries.
`matches`	object[]	required	All matches (capped at 1000).
`explanation`	object[]	required	Token-by-token explanation of the pattern.

Example request

curl -X POST https://dot.tools/api/tools/regex-tester \
  -H 'Content-Type: application/json' \
  -d '{
  "pattern": "(?<user>[\\w.+-]+)@(?<domain>[\\w-]+\\.[\\w.]+)",
  "text": "Contact us:\n  support@bitgate.com\n  sales@bitgate.com\n  kevin.schipper+tools@bitgate.com\n\nInvalid entries:\n  not-an-email\n  @missing-user.com\n  missing-at-sign.com",
  "flags": {
    "global": true,
    "ignoreCase": false,
    "multiline": true,
    "dotall": false,
    "unicode": false,
    "sticky": false
  }
}'

Sample response

{
  "ok": true,
  "result": {
    "valid": true,
    "error": null,
    "matchCount": 3,
    "truncated": false,
    "matches": [
      {
        "index": 1,
        "text": "support@bitgate.com",
        "range": [
          14,
          33
        ],
        "groups": [
          {
            "index": 1,
            "name": "user",
            "value": "support"
          },
          {
            "index": 2,
            "name": "domain",
            "value": "bitgate.com"
          }
        ]
      },
      {
        "index": 2,
        "text": "sales@bitgate.com",
        "range": [
          36,
          53
        ],
        "groups": [
          {
            "index": 1,
            "name": "user",
            "value": "sales"
          },
          {
            "index": 2,
            "name": "domain",
            "value": "bitgate.com"
          }
        ]
      },
      {
        "index": 3,
        "text": "kevin.schipper+tools@bitgate.com",
        "range": [
          56,
          88
        ],
        "groups": [
          {
            "index": 1,
            "name": "user",
            "value": "kevin.schipper+tools"
          },
          {
            "index": 2,
            "name": "domain",
            "value": "bitgate.com"
          }
        ]
      }
    ],
    "explanation": [
      {
        "token": "(?<user>",
        "description": "start of named capture group 1 \"user\""
      },
      {
        "token": "[\\w.+-]",
        "description": "any single character in this set"
      },
      {
        "token": "+",
        "description": "preceding token repeated one or more"
      },
      {
        "token": ")",
        "description": "end of group"
      },
      {
        "token": "@",
        "description": "literal \"@\""
      },
      {
        "token": "(?<domain>",
        "description": "start of named capture group 2 \"domain\""
      },
      {
        "token": "[\\w-]",
        "description": "any single character in this set"
      },
      {
        "token": "+",
        "description": "preceding token repeated one or more"
      },
      {
        "token": "\\.",
        "description": "literal \".\""
      },
      {
        "token": "[\\w.]",
        "description": "any single character in this set"
      },
      {
        "token": "+",
        "description": "preceding token repeated one or more"
      },
      {
        "token": ")",
        "description": "end of group"
      }
    ]
  }
}

Use the free RegEx Tester in your browser, read the RegEx Tester guide, or import the full OpenAPI specification.