⌘ K
every flag · side-by-side · live

Compare a regex across every flag

See match counts and first hits when only one flag is on, then compose your own combination.

Regex Flag Tester
Flags
7
compared
Active
g
1 on
Combo hits
3
matches
Sample
74 B
text
input · pattern + sample
//g
output · per-flag comparison
on
flag
count
first
name
g3"the"global
i5"The"ignoreCase
m3"the"multiline
s3"the"dotAll
u3"the"unicode
y0—sticky
d3"the"indices
Quick Start

How to compare regex flags

Run the same pattern with each JS regex flag turned on and see how matches change.

  1. 01

    Paste pattern + text

    Drop in a regex and a chunk of sample text — multi-line is fine.

    Patterns compile through the JavaScript RegExp constructor, so syntax errors surface immediately and engine quirks like Unicode property escapes (\p{L}) work when the u flag is on.

    patternECMAScriptliterallive
    input · literalpattern · regex
    the
    loaded · ECMAScriptPCRE · ready
  2. 02

    Scan per-flag rows

    Each row shows the match count and first hit when only that flag is on.

    Each flag character mirrors the ECMAScript literal: g enables iteration, i folds case, m anchors per line, s lets . match newlines, u unlocks proper Unicode, y is sticky, and d emits indices.

    flagsoptionsg · i · mlive
    row · optionscount · flags
    i → 3 hits
    flags applied · liveECMA · live
  3. 03

    Compose a combination

    Tick boxes to build a custom flag set — the combined match count updates live.

    Watch for catastrophic backtracking on nested quantifiers — a common ReDoS source. The tool caps match counts to keep the page responsive on pathological patterns.

    matchgroupshighlightReDoS-safe
    combo · outputgi · matches
    /the/gi → 3
    matches · livegroups · live
  4. 04

    Copy or export

    Grab the output as JSON, CSV, or plain text — ready for your editor or pipeline.

    Copy writes via navigator.clipboard.writeText; downloads use a Blob and object URL, so nothing transits the network and the output is byte-identical to what you see on screen.

    copyexportclipboardblob
    export · exportedready · saved
    Copied
    ready to shipsuccess
Pro tips

Regex patterns that don't melt down

Use named groups

(?<year>\d{4}) reads better than $1 and survives reordering. Supported in ECMAScript 2018+.

Anchor strictly

Use ^/$ with the m flag when matching whole lines. Without anchors, partial matches sneak through.

Avoid catastrophic backtracking

Nested quantifiers like (a+)+ can explode. Run patterns through ReDoS Detector first.

Escape every literal

Backslash any meta character you want literally: \.\$\^\*\+\?\(\)\[\]\{\}\|\\.

Test edge cases

Empty string, single character, unicode, the maximum length you expect — write fixtures for each.

Patterns aren&rsquo;t parsers

Don’t parse HTML, JSON, or balanced parens with regex. Use a proper parser when the grammar is recursive.

Related toolsRegex TesterRegex Anchor VisualizerRegex Flag ReferenceRegex DebuggerRegex ExplainerRegex Find All
Use Cases

When flag comparison helps

Pick the smallest flag set that does the job.

Teach flag semantics

Show concretely what each flag does. Compiles through the JavaScript RegExp constructor — engine quirks (Unicode property escapes, sticky matching, named groups) work when their flags are on. Error messages cite the exact position of any syntax issue.

ReDoS-safe

Audit a pattern

Confirm the right flags are on. Pairs with the ReDoS detector to catch catastrophic backtracking before you deploy. A pattern that handles 1k-character input fine can become a denial-of-service vector on attacker-controlled 1M-character input.

ECMA-compliant

Test multiline behaviour

See ^ and $ flip between modes. Output exports as JavaScript, Python re, Java util.regex, .NET System.Text.RegularExpressions, PCRE, and POSIX — language quirks are highlighted on conversion so you don't ship a JS-only pattern to a Go service.

spec-grade

Spot dotAll surprises

Confirm . crosses newlines as intended. Capture groups (named or numbered) round-trip cleanly into replacement strings, JSON output, or CSV extraction without re-typing. Backreferences ($1, ${name}) preserve semantics across languages.

cross-engine

Validate Unicode

See u flag effects on \p{...} matches. Test against edge cases — empty input, single character, max length, multibyte boundaries — to catch off-by-one and zero-length match bugs before they reach production.

test-ready

Pick the minimum

Drop flags you do not actually need. All matching runs client-side so secret patterns never leave the browser. Useful when the pattern itself is sensitive (PII regexes, internal data formats, security filters).

production-ready
FAQ

Frequently asked questions

Per-flag rows enable g so collectMatches can sweep the whole sample. The custom combo is what your final pattern would actually use.
No. JavaScript treats the flag string as a set — the order does not affect behaviour.
The d flag adds per-group start/end indices to match objects. The combined count is unaffected, but downstream code can inspect positions.
Yes. y forces matches to start at lastIndex, so most text yields fewer matches than g alone.
No. The tool runs entirely in your browser — your pattern and sample text never leave the page.
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