Regex Tester & Debugger
Test regular expressions in real-time with match highlighting, capture groups, and explanation.
About Regex Tester & Debugger
A regex (regular expression) tester allows you to write, test, and debug regular expressions in real-time against sample text. Regular expressions are powerful pattern-matching tools used for text validation, search-and-replace, data extraction, and log analysis. However, regex syntax is notoriously complex and difficult to read — this tool makes it practical to develop and refine regex patterns by showing matches, capture groups, and explanations as you type.
How to Use
Enter your regular expression pattern in the regex input field and your test text in the text area. Matches are highlighted in real-time as you type. The tool shows all matches, capture groups with their values, match positions, and an explanation of what each part of your regex pattern does. You can toggle flags like global (g), case-insensitive (i), and multiline (m).
Formula / Key Equations
Regular expression syntax includes: character classes [a-z], quantifiers (*, +, ?, {n,m}), anchors (^, $), groups ((...)), alternation (|), escapes (\d, \w, \s), lookaheads (?=...), lookbehinds (?<=...), and backreferences (\1). Flags modify behavior: g (global), i (case-insensitive), m (multiline), s (dotAll).
Common Use Cases
Validating email addresses, phone numbers, and other user input formats. Extracting data from log files or CSV documents. Search and replace in text editors and IDEs. Building URL routing patterns in web frameworks. Parsing structured text data. Creating input validation rules for web forms.
Limitations
The tool uses JavaScript's RegExp engine, which may behave differently from regex engines in Python (re), PCRE, or other languages. Backreferences, lookbehinds, and Unicode handling can vary between engines. Very complex regex patterns may cause performance issues (catastrophic backtracking). The explanation feature covers common patterns but may not explain every edge case.
Frequently Asked Questions
What is a regular expression?
A regular expression (regex) is a sequence of characters that defines a search pattern for text. It can match simple strings ('hello'), character classes ('[0-9]+'), or complex patterns ('^\d{3}-\d{2}-\d{4}$' for SSN format). Regex is supported in virtually every programming language and text processing tool.
How do capture groups work?
Capture groups, denoted by parentheses (...), capture the matched text for later use (backreference or extraction). For example, the pattern '(\d+)-(\d+)' matching '2024-01' captures '2024' in group 1 and '01' in group 2. Named groups (?P<name>...) make extraction more readable.
What is catastrophic backtracking?
Catastrophic backtracking occurs when a regex pattern has nested quantifiers that can match the same text in multiple ways, causing the engine to explore exponentially many possibilities. For example, '(a+)+b' matching 'aaaaaaaaaaaaaaaaaaaaac' will try every combination before failing. Use possessive quantifiers or atomic groups to prevent this.
Why does my regex work differently in Python vs JavaScript?
Regex engines differ in features. JavaScript uses a NFA engine similar to PCRE but lacks some features like lookbehinds (until recently), named groups, and Unicode property escapes. Python's re module supports named groups and some features JS lacks. Always test regex in the target language's engine.
How do I test regex with multiline text?
Use the multiline flag (m) which makes ^ and $ match the start and end of each line (not just the start/end of the entire string). Without this flag, ^ and $ only match the beginning and end of the entire input. Our tool lets you toggle this flag with one click.
Related Tools
JSON Formatter & Validator
Format, beautify, minify, and validate JSON data w...
Base64 Encode / Decode
Encode text to Base64 or decode Base64 to text. Su...
URL Encode / Decode
Encode or decode URLs and query parameters. Handle...
HTML to Markdown Converter
Convert HTML to clean Markdown or Markdown to HTML...
Code Diff Checker
Compare two code snippets side-by-side with highli...