Regex Tester

About Regex tester

The regex tester runs a regular expression against your sample text and highlights every match as you type. It reports match positions, capture groups and named groups so you can confirm a pattern does what you meant before it goes into code. It uses your browser's JavaScript engine, which makes it the right check for JavaScript and TypeScript work.

How to use it

1. Enter the pattern in the Pattern field without surrounding slashes.
2. Toggle flag buttons g, i, m, s, u and d as needed. Only g is on to begin with.
3. Paste your sample into the Test string box and watch the count badge and the Match preview highlighting.
4. Open the Match details list to see each match's start and end index and its groups, with a copy button on every row.
5. Expand Quick reference and click any token to insert it at the cursor in the pattern.

Named groups on ISO-style dates, flag g

Pattern: (?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})
Test string: Released 2024-03-15, patched 2024-04-02.

2 matches. #1 2024-03-15 at 9-19 (year 2024, month 03, day 15). #2 2024-04-02 at 29-39 (year 2024, month 04, day 02).

When to use it

• Tuning a validation pattern for form input until it accepts good values and rejects bad ones.
• Extracting fields from log lines by checking what each capture group returns.
• Explaining a colleague's pattern by watching which parts of a sample it highlights.
• Checking how flags such as multiline or dot-all change where a pattern matches.

Questions about Regex tester

Which regex flavor does it use?

The JavaScript engine built into your browser. Possessive quantifiers, atomic groups and a leading inline flag such as (?i) show a syntax error. The \A and \Z anchors are subtler: without the u flag they are accepted as identity escapes and silently match a plain letter A or Z, while with u they are an error. Very recent engines do accept a scoped group like (?i:abc).

Is there a limit on input size?

Only the first 50,000 characters of the test string are analyzed, and a notice says so. With the g flag, matching stops after 10,000 matches to keep the page responsive.

Why is an error shown as I type?

The pattern is rebuilt about 150 milliseconds after each change, and half-typed patterns, such as an unclosed bracket, are invalid until finished. The badge reads Invalid regex and the engine's message appears under the field.

Can a pattern freeze the tab?

Yes, if it has nested quantifiers like (a+)+ against a long non-matching string, because backtracking can grow exponentially. Simplify the pattern or shorten the sample if the page stops responding.

What does the d flag add?

It asks the engine to generate match indices. The tool already reports start and end positions for each match, so d mainly matters if you plan to reuse the same flag string in your own code.

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