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Regex Tester

Test and debug regular expressions with live matching and highlighting. See all matches, capture groups, and index positions instantly as you type.

Developer Tool
//g

Enter a regex pattern and test string above to see matches in real time

How to Use

  1. 1Enter your regular expression pattern in the pattern field.
  2. 2Set regex flags as needed: g (global), i (case-insensitive), m (multiline).
  3. 3Paste or type test strings in the input area.
  4. 4Matches are highlighted in real time as you type, showing what your pattern captures.
  5. 5Refine your pattern until it matches exactly what you intend.

About This Tool

The Regex Tester provides live pattern matching with visual highlighting so you can build and debug regular expressions interactively. Type a pattern, enter test text, and instantly see what matches — no need to write code, run scripts, or reload a page.

Regular expressions are powerful but notoriously difficult to get right. A pattern that looks correct can fail on edge cases: unexpected whitespace, Unicode characters, greedy vs. lazy matching, or escaped special characters. Testing in a dedicated tool catches these issues before the pattern reaches your codebase.

The live highlighting makes the matching behavior visible. You can see exactly which characters are captured, where groups start and end, and which parts of your test text are not matching. This visual feedback is far more informative than a boolean match/no-match result.

Common use cases include validating email addresses, parsing log files, extracting data from structured text, and building search-and-replace patterns. The tool supports standard JavaScript regex syntax and flags. For patterns used in other languages (Python, Java, Go), note that flag syntax and some features differ slightly.

Tips & Best Practices

  • ✓Start with a simple pattern that matches too broadly, then narrow it down. Building a regex from specific to general is harder and leads to more errors.
  • ✓Use non-greedy quantifiers (.*? instead of .*) when matching content between delimiters — greedy matching will grab everything up to the LAST occurrence, not the first.
  • ✓Test edge cases explicitly: empty strings, strings with only whitespace, strings with special characters, and very long strings. Production data always contains surprises.

Frequently Asked Questions

What is a regular expression (regex)?▼
A regular expression is a sequence of characters that defines a search pattern. Regular expressions are used to match, search, and manipulate text in virtually every programming language. Common uses include input validation (email, phone number), text search and replace, log parsing, and data extraction.
What are common regex patterns for email, phone, and URL?▼
Common patterns include: Email — /^[\w.-]+@[\w.-]+\.[a-zA-Z]{2,}$/ ; Phone (US) — /^\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}$/ ; URL — /https?:\/\/[\w.-]+(?:\.[a-zA-Z]{2,})(?:\/[^\s]*)?/ . These are simplified patterns; production use may require more comprehensive versions.
What are capture groups and how do they work?▼
Capture groups are defined by parentheses () in a regex pattern. They capture the matched text so you can extract or reference it later. For example, the pattern /(\d{4})-(\d{2})-(\d{2})/ applied to "2024-01-15" captures three groups: "2024", "01", and "15". Named groups use the syntax (?<name>...) for clearer references.
What are lookaheads and lookbehinds in regex?▼
Lookaheads and lookbehinds are zero-width assertions that match a position without consuming characters. A positive lookahead (?=...) asserts that what follows matches the pattern. A negative lookahead (?!...) asserts that what follows does not match. Lookbehinds (?<=...) and (?<!...) work the same way but check what precedes the current position. They are useful for complex matching without including surrounding text in the result.
How can I optimize regex performance?▼
To improve regex performance: (1) Be specific — avoid .* when you can use a character class like [^"]*. (2) Use non-capturing groups (?:...) when you do not need the captured text. (3) Avoid nested quantifiers like (a+)+ which can cause catastrophic backtracking. (4) Anchor your patterns with ^ and $ when possible. (5) Use possessive quantifiers or atomic groups if your engine supports them.
What are best practices for testing regular expressions?▼
Best practices include: (1) Start with simple patterns and build up complexity gradually. (2) Test with both matching and non-matching inputs to verify the pattern rejects invalid text. (3) Test edge cases like empty strings, very long strings, and special characters. (4) Use the global (g) flag to find all matches. (5) Document complex patterns with comments or named groups. (6) Use a tool like this regex tester to iterate quickly before embedding patterns in code.

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