Regex Tester & Debugger
Test, debug, and optimize regular expressions in real-time. Live match highlighting, capture group analysis, flag toggles, and string replacement.
Regular Expression Pattern
//
Test String Input
Matches & Capture Groups
ActiveMatch #1: “[email protected]”Index: 8
Match #2: “[email protected]”Index: 35
Replacement Preview Result
Contact [email-redacted] or [email-redacted] for inquiries. Invalid: [email protected], test@domain
How to Use the Regular Expression Tester
- Input Your Regex Pattern: Enter a pattern between the forward slashes (e.g.
[a-zA-Z0-9]+), or pick a common preset (Email, URL, IP address). - Toggle Behavior Flags: Click g (global match), i (case-insensitive), m (multiline), or s (dotAll) to configure evaluation parameters.
- Paste Test Strings: Enter sample text into the editor. The tool compiles the regex in real time and reports exact match counts.
- Analyze Capture Groups: Check the match table to inspect sub-captures (
$1,$2) and character index offsets. - Test String Replacements: Enter a replacement expression (e.g.
$2, $1or[redacted]) to preview replacements without altering original code.
Quick Regex Syntax Cheat Sheet
Character Classes
.- Any character except newline\d/\D- Digit (0-9) / Non-digit\w/\W- Word character (a-z, A-Z, 0-9, _) / Non-word\s/\S- Whitespace (space, tab, newline) / Non-whitespace[a-z]- Any character in range a to z[^0-9]- Negated character class (not 0-9)
Quantifiers & Anchors
*- 0 or more (greedy)+- 1 or more (greedy)?- 0 or 1, or lazy modifier (e.g..*?){3,5}- Between 3 and 5 times^/$- Start of string / End of string\b- Word boundary
Why Test Regular Expressions in Real-Time?
- Prevent Catastrophic Backtracking (ReDoS): Unintentional nested quantifiers (such as
(a+)+$) cause CPU hangs in production servers. Interactive testing catches inefficient patterns early. - Greedy vs. Lazy Debugging: Visualizing exactly what gets matched helps prevent greedy quantifiers from eating closing delimiters or tags.
- 100% Client-Side Privacy: Test customer PII, internal log strings, and sensitive credentials safely without sending data over the network.