Why does a regex match locally but fail in production?

Check flags (g/i/m), escaping differences between string literals and regex literals, multiline input handling, and runtime engine differences (JavaScript vs Python vs PCRE). Copy the exact sample and options into Regex Tester before changing the pattern.

What's the difference between g, gi, and gm flags?

g (global) finds all matches instead of stopping at the first. gi (global + case-insensitive) ignores letter casing. gm (global + multiline) makes ^ and $ match line boundaries instead of string boundaries.

How do I match a literal dot (.) character?

Escape it with a backslash: \. Unescaped . matches any character except newline. In the input field, enter the backslash literally (not as an escape sequence).

Can I test lookbehind and lookahead assertions?

Yes. Positive lookahead: (?=...), negative lookahead: (?!...). Positive lookbehind: (?<=...), negative lookbehind: (?<!...). Note: some older browsers have limited support for lookbehind.

Which tools pair well with Regex Tester?

Common next steps include Diff Checker, URL Encode/Decode. These related tools help continue the same workflow through generation, validation, formatting, or debugging.

Last reviewed: 2026-05-15

Regex Tester quick answer

Regex Tester is a browser-first tool for test patterns live with capture groups.. It is designed for fast one-off work without account setup.

Use it when you need to test patterns live with capture groups.
Use it when you need to regex
Use it when you need to tester

This text tool is local-first in the browser and does not store your input for normal conversion workflows.

Regex Tester returns a copyable result that can be reviewed immediately or continued in a related ToolMeshly workflow.

About Regex Tester

Test regular expressions in real-time with capture group highlighting and match count.

Enter the text you want to search in the left textarea.
Enter your regular expression pattern in the right textarea.
Select the appropriate regex flags: Default, g, gi, or gm.
Click Test to see the matches highlighted.
Review the match count in the output panel.
Inspect capture groups for detailed matches.

FAQ

What is Regex Tester best for?: Test regular expressions in real-time with capture group highlighting and match count.
Does Regex Tester upload or store my data?: This text tool is local-first in the browser and does not store your input for normal conversion workflows.
Why does a regex match locally but fail in production?: Check flags (g/i/m), escaping differences between string literals and regex literals, multiline input handling, and runtime engine differences (JavaScript vs Python vs PCRE). Copy the exact sample and options into Regex Tester before changing the pattern.
What's the difference between g, gi, and gm flags?: g (global) finds all matches instead of stopping at the first. gi (global + case-insensitive) ignores letter casing. gm (global + multiline) makes ^ and $ match line boundaries instead of string boundaries.
How do I match a literal dot (.) character?: Escape it with a backslash: \. Unescaped . matches any character except newline. In the input field, enter the backslash literally (not as an escape sequence).
Can I test lookbehind and lookahead assertions?: Yes. Positive lookahead: (?=...), negative lookahead: (?!...). Positive lookbehind: (?<=...), negative lookbehind: (?<!...). Note: some older browsers have limited support for lookbehind.
Which tools pair well with Regex Tester?: Common next steps include Diff Checker, URL Encode/Decode. These related tools help continue the same workflow through generation, validation, formatting, or debugging.

Related Tools

Diff Checker URL Encode/Decode

Guides & tutorials

Regex Tester

Local processing

Test patterns live with capture groups.

Input

Pattern

Output