Regular Expression Tester

What is a Regular Expression?

A regular expression (regex or regexp) is a sequence of characters that defines a search pattern. Regular expressions are used in programming, text editors, and command-line tools to find, match, validate, and manipulate strings of text. They provide a concise and flexible way to identify patterns such as email addresses, phone numbers, URLs, dates, and virtually any structured text format.

Originally developed in the 1950s by mathematician Stephen Cole Kleene, regular expressions are now a fundamental tool in every developer's toolkit. They are supported natively by JavaScript, Python, Java, PHP, Ruby, Go, and virtually every modern programming language.

How to Use This Tool

1. Enter your regex pattern in the pattern input field at the top.
2. Select the flags you need: Global (g) to match all occurrences, Case-insensitive (i), Multiline (m) to match across lines, or Dotall (s) to make the dot match newlines.
3. Paste or type the sample text you want to test against.
4. In Match mode, all matches are highlighted inline within your text, and any capture groups are shown in a detailed table.
5. Switch to Replace mode to enter a replacement pattern and see the live result. Use $1, $2, etc. for group back-references.
6. Use the Cheat Sheet to quickly load common patterns like email, URL, IP address, phone number, date, or hex color.

Common Use Cases

• Form validation — verify that user input matches expected formats like emails, phone numbers, or postal codes before submission.
• Log parsing — extract timestamps, error codes, IP addresses, or user agents from server log files.
• Data extraction — pull structured data from unstructured text, such as prices from product descriptions or dates from documents.
• Search and replace — perform sophisticated find-and-replace operations across codebases, using capture groups to restructure matched content.
• Input sanitization — strip or escape unwanted characters from user input to prevent injection attacks or formatting errors.

Tips and Common Mistakes

• Escape special characters — characters like ., *,+, ?, (, ), [,], {}, |, \, and ^ have special meaning. Prefix them with a backslash to match them literally.
• Avoid greedy traps — quantifiers like .* are greedy by default and will match as much text as possible. Use .*? for non-greedy (lazy) matching.
• Anchor your patterns — use ^ and $ to match the start and end of a string. Without anchors, partial matches may pass validation.
• Test edge cases — always test with empty strings, very long strings, special characters, and unicode text to make sure your pattern handles them correctly.
• Use non-capturing groups — if you need grouping but not capture, use (?:...) instead of (...) for better performance.
• Be careful with backtracking — complex nested quantifiers like (a+)+ can cause catastrophic backtracking. Keep patterns simple and specific.