Testing

Regular Expression (RegEx) Tester & Debugger

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Regular Expression Pattern & Flags

Valid Pattern (1.24 ms)

//gi

Presets:

Test String Textarea

🔄 Replacement Pattern ($1, $2, $<name>)

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Testing Guide

About the Regular Expression (RegEx) Tester & Debugger

Regular expressions provide unmatched pattern-matching power for input validation, data extraction, log scraping, and string transformations. However, crafting complex expressions involving lookarounds, non-capturing groups, and quantifiers often leads to subtle logic bugs or catastrophic backtracking without real-time visual inspection. This live RegEx studio compiles and executes JavaScript regular expressions directly in your browser. Inspect highlighted match ranges, examine named and numbered capture groups in an organized breakdown table, test string substitutions with backreferences ($1, $<name>), and browse a built-in library of common production patterns.

In-Depth Technical Guide

How Regular Expression (RegEx) Tester & DebuggerWorks & What the Results Mean

How JavaScript Regular Expression Engines Evaluate Patterns

The JavaScript ECMAScript RegEx engine utilizes a deterministic finite automaton (DFA) and non-deterministic finite automaton (NFA) backtracking model to match strings against regular expression tokens. Understanding how key tokens interact is crucial for writing efficient patterns:

Character Classes & Metacharacters: Shorthands like \d (digits [0-9]), \w (alphanumerics and underscore [A-Za-z0-9_]), and \s (whitespace) match specific token classes.
Quantifiers: Greedy quantifiers (+, *, {min,max}) consume as many characters as possible before backtracking. Adding a lazy modifier (+?, *?) forces the engine to match the minimum necessary characters.
Anchors & Boundaries: ^ asserts the start of a string (or line in multiline mode), $ asserts the end, and \b establishes word boundaries.
Lookarounds: Positive lookaheads (?=...) and negative lookaheads (?!...), along with positive lookbehinds (?<=...) and negative lookbehinds (?<!...), assert conditions without consuming characters in the match result.

Working with Named and Numbered Capture Groups

Parentheses (...) create capture groups that isolate specific substrings from the overall match. Numbered capture groups are indexed from $1 to $99 based on opening parenthesis order. Modern JavaScript (ES2018+) also supports named capture groups using the (?<name>...) syntax. Named groups enhance code readability by returning structured key-value dictionaries, making parameter extraction from URLs and log lines intuitive and self-documenting.

Guarding Against Catastrophic Backtracking (ReDoS)

Regular Expression Denial of Service (ReDoS) occurs when nested quantifiers (e.g. (a+)+$) cause the engine to explore exponential execution paths when presented with non-matching inputs. Our testing environment implements client-side execution timeouts to catch runaway backtracking loops before they can freeze your browser tab.

Step-by-Step Guide

How to Use Regular Expression (RegEx) Tester & Debugger

1Type your regular expression pattern into the dedicated Pattern input bar.
2Toggle your desired RegEx flags: Global (g), Case-Insensitive (i), Multiline (m), DotAll (s), or Unicode (u).
3Enter your test data or sample log lines into the Test String text area.
4Observe the color-coded match highlights and real-time Match Count counter.
5Review the Capture Groups breakdown table showing group index numbers, group names, and extracted values.
6Switch to Replace Mode to test string substitution using capture backreferences ($1, $2, $<name>).

Capabilities

Key Features & Highlights

Real-time visual syntax highlighting for all matching character spans and boundary tokens.
Detailed capture groups table displaying group indices, custom group names, and extracted substrings.
Full flag configuration: 'g' (global), 'i' (ignore case), 'm' (multiline), 's' (dotAll), 'u' (unicode).
Built-in Cheat Sheet library of ready-to-use patterns (Email, URL, IP Address, Date, Phone, HTML Tags).
ReDoS (Regular Expression Denial of Service) timeout safeguard preventing catastrophic backtracking.
Replace mode supporting capture backreferences ($1, $2, $<name>) with instant output preview.
Support for modern JavaScript regex capabilities (Named Capture Groups, Lookbehinds, Unicode properties).
100% in-browser evaluation with instant updates as you type.

Practical Scenarios

Examples & Real-World Use Cases

Validating Email Addresses

Scenario: Extracting valid email addresses from unstructured contact logs.

Sample Input:

Pattern: [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,} | Flags: gi

Expected Output:

Matches: 'support@antigravity.dev', 'admin@example.org'

Isolates RFC-compliant email address strings.

Extracting URL Query Parameters with Named Groups

Scenario: Capturing key-value pairs using named capture groups.

Sample Input:

Pattern: (?<key>\w+)=(?<val>[^&]+) | String: 'page=2&sort=desc'

Expected Output:

Group 'key': 'page', Group 'val': '2' | Group 'key': 'sort', Group 'val': 'desc'

Extracts structured parameter dictionaries from URL query strings.

Common Questions

Frequently Asked Questions

Greedy quantifiers (, +, {n,}) match as much text as possible before allowing subsequent pattern tokens to match. Appending a question mark (?, +?, {n,}?) makes the quantifier lazy, causing it to match the smallest possible substring.

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