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Free Regex Tester

Validate JavaScript regex patterns against sample text, inspect capture groups, and debug no-match cases quickly.

Copy-paste outputs

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Tool 01

Regex Tester

Test regex patterns with flags, inspect capture groups, and debug no-match edge cases.

Regex Tester

Test JavaScript regular expressions with flags and capture group inspection.

Pattern

Flags

Input text

Matches (2)

[
  {
    "match": "Brand Armor",
    "index": 0,
    "groups": []
  },
  {
    "match": "brand armor",
    "index": 38,
    "groups": []
  }
]

How it works

Regex Tester: methodology and worked example

How this tool computes its result

The tool constructs `new RegExp(pattern, flags)` directly from your input and runs it against the test string using the native JavaScript regex engine. When the "g" flag is present, it loops calling regex.exec(input) repeatedly, manually advancing regex.lastIndex by 1 whenever a match is empty-string (to avoid an infinite loop on patterns like `x*`), collecting each match's full text, start index, and capture groups. Without "g", it runs exec() once and returns at most one match.

Worked example

Pattern `\bbrand\s+armor\b`, flags "gi", input "Brand Armor AI helps marketing teams. brand armor visibility matters.": two matches are found — {match: "Brand Armor", index: 0, groups: []} and {match: "brand armor", index: 38, groups: []} — both case-insensitive word-boundary matches on the literal phrase.

When not to use this tool

This runs the browser's native JS regex engine only — it does not emulate PCRE, POSIX, Python `re`, or other dialects, so a pattern copied from another language's docs may behave differently here or throw a SyntaxError. There is also no catastrophic-backtracking protection: a pathological pattern (e.g. nested quantifiers) run against a long input can hang the tab, since it executes synchronously in the render path.

Common mistakes

- Forgetting the "g" flag when expecting all occurrences — without it, exec() returns only the first match and the "Matches" count stays at 1 (or 0) regardless of how many times the pattern actually appears.
- Writing an invalid pattern or duplicate/unsupported flag characters — the try/catch surfaces the native SyntaxError message (e.g. "Invalid flags supplied to RegExp constructor") rather than a friendlier explanation.
- Assuming capture groups are always populated — `groups: m.slice(1)` will be an empty array for any pattern with no parenthesized groups, not an error.

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