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Regex Tester

Test regular expressions in real-time with match highlighting, group capture, and flag controls.

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2 matches

Matches

 
 
     
 
 
 
 
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What Is a Regex Tester?
 

A regex tester is an interactive tool that helps developers write, test, and debug
        regular expressions (regex) in real-time. It shows exactly which parts of your input text match
        your pattern, making it invaluable for building and troubleshooting regex patterns for validation,
        search, parsing, and text transformation tasks.

 
Common Regex Patterns
 
 
 
Email
 \w+@\w+\.\w+ 
 
 
Phone (US)
 \(\d3\)\s\d3-\d4 
 
 
URL
 https?://\S+ 
 
 
IP Address
 \d3\.\d3\.\d3\.\d3 
 
 
Understanding Regex Flags
 
 
 
g — Global
 
Find all matches, not just the first one.
 
 
 
i — Case-Insensitive
 
Ignore case when matching letters.
 
 
 
m — Multiline
 
^ and $ match line boundaries, not string boundaries.
 
 
 
s — Dotall
 
The . metacharacter matches newlines too.
 
 
 
Use Cases for Regex Testing
 
     
  • Form Validation — Validate email addresses, phone numbers, URLs, and postal codes in web forms.
    •  
  • Log Parsing — Extract structured data from server logs, application logs, and error reports.
    •  
  • Data Extraction — Pull specific patterns from large text documents, API responses, or configuration files.
    •  
  • Search & Replace — Find and transform text patterns across files with regex-based substitution.
    •  
  • Input Sanitization — Filter and clean user input by matching allowed or disallowed patterns.
    •  
 
How to Use This Regex Tester
 

Enter your regex pattern between the forward slashes in the pattern input field. Add flags (g, i, m, s, u)
        using the flag buttons or the flags input. Type or paste your test string in the text area below.
        Matches are highlighted in real-time with colored backgrounds, and capturing groups are displayed
        in the groups section below the matches. If your regex is invalid, an error message will show
        the syntax problem.

 
 
 
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Regular expressions (regex) are formal pattern-matching languages used across software engineering for data validation, string parsing, text extraction, and automated search-and-replace. Despite their power, crafting regular expressions is notoriously tricky: subtle distinctions between greedy (`*`) and lazy (`*?`) quantifiers, character classes (`\d`, `\w`, `\s`), boundary anchors (`^`, `$`, `\b`), and lookarounds (`(?=...)`, `(?<=...)`) can lead to unexpected matching bugs or catastrophic backtracking. Catastrophic backtracking, also known as Regular Expression Denial of Service (ReDoS), occurs when an unanchored pattern with overlapping nested quantifiers causes an exponential number of search paths, freezing CPU cores. A real-time regex tester provides an interactive feedback loop: as you type your pattern, matches are highlighted instantly in your sample text, capture groups are parsed into structured results, and flags (`g`, `i`, `m`, `s`, `u`) can be toggled effortlessly. WebUtil's Regex Tester runs directly in your browser with built-in execution limits to protect against runaway loops, ensuring safe, rapid regex development.
 
 
 
 
 
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Related Guides & Tutorials
 
  
 Text 6 min read 
 
Regex Tutorial for Beginners: Learn Regular Expressions with Examples
 
A complete regex tutorial for beginners. Learn how to use regular expressions for pattern matching, text search, and data validation with practical examples.
  
 
 
 
 
Regex Tester FAQ
 
 
  What regex engine does this online tester use?     
WebUtil uses the ECMAScript (JavaScript) RegExp engine built into modern web browsers, supporting modern features like named capture groups, unicode property escapes, and lookbehinds.
 
  What do the regex flags (g, i, m, s, u) mean?     
g = global (find all matches), i = case-insensitive, m = multiline (^ and $ match line boundaries), s = dotAll (. matches newlines), and u = unicode (treat pattern as unicode code points).
 
  What is Regular Expression Denial of Service (ReDoS) and how is it prevented?     
ReDoS happens when complex patterns with nested quantifiers cause exponential backtracking on non-matching strings, freezing CPU cores. WebUtil includes safety execution limits to prevent browser tab lockups.
 
  How do lookahead and lookbehind assertions work in JavaScript regex?     
Lookahead (?=...) checks if a pattern follows without including it in the match. Lookbehind (?<=...) checks if a pattern precedes. Negative assertions (?!...) and (?<!...) ensure patterns do NOT match.
 
  Can I test regex substitution and replacement patterns ($1, $2)?     
Yes. You can enter a replacement string using $1, $2, or named group references $<name> to preview string replacements in real time.
 
  Why does my regex work in Python or PCRE but fail in JavaScript?     
Different languages use different regex engines. While JavaScript now supports lookbehinds and named groups, features like possessive quantifiers (*+) and atomic grouping ((?>...)) are unique to PCRE/Java.