Why does the tool highlight every match even when I haven't checked the g (global) flag?

The preview always searches globally internally so it can find and mark every match in the text. The g checkbox mainly reflects what your own code would see if it used this exact pattern outside the tool.

Why don't some matches show up highlighted even though they're in the match count?

Zero-length matches, the kind a lookahead or word-boundary pattern alone can produce, are skipped in the highlighted view since there's no text to wrap in a highlight. They're still counted as real matches.

Regex Tester Online: Test & Debug Regular Expressions

3 matches

Test stringpaste or type · or drop a .txt file

Browse

Highlighted3 matches
Contact: alice@example.com or bob@dev.org — billing@company.co.uk

Matches

1alice@example.com@ 9

2bob@dev.org@ 30

3billing@company.co.uk@ 44

Flags: g, i, m, s explained

Combining flags like gi is common for case-insensitive global search.

gGlobal — finds all matches, not just the first

iCase-insensitive matching

mMultiline — ^ and $ match line boundaries

sDotAll — . also matches newline characters

Capture groups and named groups

Wrap part of a pattern in parentheses to create a capture group: (\d+) captures digits. The captured text appears in $1, $2, etc.

Named groups (?<year>\d{4}) give the capture a label accessible via the match.groups object. Non-capturing groups (?:...) group for quantifiers without capturing. This tool shows all captured groups below each match.

Common regex patterns for developers

These are starting points; production validation usually needs stricter patterns or a dedicated library.

Email^[\w.-]+@[\w.-]+\.[a-z]{2,}$

UUID[0-9a-f]{8}-([0-9a-f]{4}-){3}[0-9a-f]{12}

ISO date\d{4}-\d{2}-\d{2}

IP address(\d{1,3}\.){3}\d{1,3}

Semver\d+\.\d+\.\d+

Frequently asked questions

This tool runs your pattern through JavaScript's native RegExp engine, the same one your browser uses. Syntax tied to other engines, PCRE-style possessive quantifiers and atomic groups, or Go RE2's lack of backreferences, won't behave the same way here. Treat this as a JavaScript-behavior check, not a universal validator across languages.