Regular Expression Tester

Test and debug regex patterns with real-time matching, capture groups, and detailed match positions.

Regular Expression

/ / gi

Examples:

Flags

g global i ignore case m multiline s dotall

Test String

Quick Reference

. Any character

\d Digit [0-9]

\w Word char [a-zA-Z0-9_]

\s Whitespace

^ Start of string

$ End of string

* Zero or more

+ One or more

? Zero or one

{n} Exactly n times

[abc] Character set

(...) Capture group

Regex Flags Explained

g (global) — Find all matches in the string, not just the first one. Without this flag, only the first match is returned.

i (ignore case) — Make the pattern case-insensitive. "A" matches "a" and vice versa.

m (multiline) — Treat ^ and $ as line boundaries instead of string boundaries. Useful for multi-line text.

s (dotall) — Allow . to match newline characters. Without this flag, . matches everything except newlines.

Global vs Single Match

Without the g flag, the regex returns only the first match with detailed capture group information. This is useful when you need to extract specific parts of a single match.

With the g flag, the regex finds all matches in the string. Each match shows its position (start and end index) so you can locate it in your original text.

For patterns with capture groups, use g flag with matchAll() behavior to see groups for each match.

Capture Groups and What They Return

Capture groups are created with parentheses () in your pattern. They let you extract specific parts of a match.

Example: Pattern (\d{4})-(\d{2})-(\d{2}) matching "2024-03-15" returns:

Group 0 (full match): "2024-03-15"
Group 1: "2024" (year)
Group 2: "03" (month)
Group 3: "15" (day)

Groups that don't match anything return undefined. Optional groups with ? may be empty.

Common Regex Mistakes

Forgetting to escape special characters — Characters like . * + ? ^ $ { } [ ] \ | ( ) need escaping with \ when used literally
Greedy vs lazy quantifiers — Use *? or +? for lazy matching to avoid matching too much
Wrong character class syntax — Inside [ ], most special characters don't need escaping except ] and \
Missing anchors — Use ^ and $ to match the entire string, not just a part
Not using raw strings — In some languages, you need double escaping: \\d instead of \d

How to Test a Pattern Step by Step

Start simple — Test with a basic pattern first, then add complexity
Use flags appropriately — Add 'i' for case-insensitive, 'g' for multiple matches
Check each capture group — Verify groups capture what you expect
Test edge cases — Try empty strings, special characters, and boundary conditions
Verify positions — Check start/end indices to ensure matches are where you expect

Frequently Asked Questions

Is my regex data sent to a server? ▼

No. All regex testing happens directly in your browser. Your patterns and test strings never leave your device.

Why does my pattern match but show empty groups? ▼

Capture groups that contain optional patterns (with ?) may match nothing. Also, groups inside quantifiers like (a)* only capture the last iteration. Use non-capturing groups (?:...) if you don't need to extract that part.

What's the difference between [ ] and ( )? ▼

Square brackets [ ] define a character set — match any one character inside. Parentheses ( ) create a capture group — match the entire sequence and remember it for later extraction.

How do I match a literal dot or question mark? ▼

Escape special characters with a backslash: \. for a literal dot, \? for a literal question mark, \* for a literal asterisk, and so on.