What are Regular Expressions?
Regular expressions (regex or regexp) are powerful pattern-matching tools used to search, match, and manipulate text. They provide a concise and flexible way to identify strings of text, such as particular characters, words, or patterns of characters. Mastering regex is an essential skill for developers, data analysts, and anyone working with text processing.
A regex pattern is a sequence of characters that defines a search pattern. When applied to text, the regex engine attempts to find matches according to the rules defined in the pattern. The power of regex lies in its ability to express complex matching rules in a compact notation.
Understanding Regex Syntax
Character Classes
Character classes let you match any one of several characters at a specific position.
.(dot) - Matches any single character except newline\d- Matches any digit (0-9)\D- Matches any non-digit character\w- Matches word characters (letters, digits, underscore)\W- Matches non-word characters\s- Matches whitespace (space, tab, newline)\S- Matches non-whitespace characters[abc]- Matches any character in the brackets[^abc]- Matches any character NOT in the brackets[a-z]- Matches any character in the range
Quantifiers
Quantifiers specify how many times a character or group should occur.
*- Zero or more times+- One or more times?- Zero or one time (optional){n}- Exactly n times{n,}- n or more times{n,m}- Between n and m times
Anchors
Anchors match positions in the string rather than characters.
^- Start of string (or line in multiline mode)$- End of string (or line in multiline mode)\b- Word boundary\B- Non-word boundary
Groups and Backreferences
Groups allow you to treat multiple characters as a single unit.
(...)- Capturing group(?:...)- Non-capturing group(?<name>...)- Named capturing group\1, \2- Backreference to captured groups(?=...)- Positive lookahead(?!...)- Negative lookahead
How to Use This Tool
Enter your pattern: Type your regex pattern in the input field. The tool validates the pattern in real-time and shows any syntax errors immediately.
Configure flags: Toggle the regex flags according to your needs:
g(global) - Find all matches, not just the firsti(case insensitive) - Ignore case when matchingm(multiline) - ^ and $ match line boundariess(dotall) - Dot matches newlines toou(unicode) - Enable full Unicode support
Enter test text: Type or paste the text you want to test against your pattern. Matches are highlighted in real-time with different colors.
Choose a mode:
- Test mode shows highlighted matches
- Replace mode lets you substitute matches
- Extract mode lists all matches with group details
Common Regex Patterns
Email Validation
Pattern: [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}
Matches most valid email addresses. For production use, consider more comprehensive patterns or use built-in validation.
URL Matching
Pattern: https?:\/\/[\w.-]+(?:\.[\w.-]+)+[\w\-._~:/?#[\]@!$&'()*+,;=]*
Matches HTTP and HTTPS URLs with various path components.
Phone Numbers (US)
Pattern: \(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}
Matches common US phone number formats like (555) 123-4567, 555.123.4567, or 555-123-4567.
IP Addresses
Pattern: \b(?:(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\b
Validates IPv4 addresses, ensuring each octet is in the valid range 0-255.
Dates (YYYY-MM-DD)
Pattern: \d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])
Matches ISO format dates with basic validation for month and day ranges.
Regex Best Practices
Start simple: Begin with a basic pattern and add complexity as needed. Test frequently to ensure each addition works correctly.
Be specific: Avoid overly broad patterns that might match unintended text. Use character classes and anchors to be precise.
Use non-capturing groups: When you need grouping for quantifiers but don’t need the captured value, use (?:...) instead of (...) to improve performance.
Test edge cases: Always test with empty strings, very long strings, and unusual characters to ensure your pattern handles all scenarios.
Comment complex patterns: In languages that support it, use verbose mode to add comments explaining complex patterns.
Regex in Different Languages
While regex syntax is largely standardized, different programming languages have slight variations:
JavaScript: Uses forward slashes as delimiters (/pattern/flags). Supports named groups with (?<name>...).
Python: Uses raw strings (r"pattern") to avoid escape issues. The re module provides regex functions.
PHP: Requires delimiters like /pattern/flags with preg functions.
Java: Uses double backslashes for escapes (\\d instead of \d). Pattern and Matcher classes provide regex functionality.
Performance Considerations
Regex can be computationally expensive, especially with certain patterns. Be cautious with:
- Nested quantifiers like
(a+)+ - Overlapping alternatives like
(a|a)+ - Excessive backtracking from greedy quantifiers
For performance-critical applications, consider pre-compiling patterns and testing with realistic data volumes.
Privacy and Security
All regex testing happens entirely in your browser using native JavaScript. Your patterns and test data are never sent to any server. This makes the tool safe for testing patterns with sensitive information.
Frequently Asked Questions
Why isn’t my regex matching anything?
Check that you’ve enabled the global flag (g) if you want to find multiple matches. Also verify your pattern syntax—common issues include missing escape characters for special characters.
What’s the difference between * and +?
* matches zero or more occurrences (so the pattern can match even if that part is missing), while + requires at least one occurrence.
How do I match a literal dot or other special characters?
Escape special characters with a backslash: \. for dot, \* for asterisk, \\ for backslash itself.
Why are some patterns slow?
Patterns with nested quantifiers or excessive backtracking can cause exponential time complexity. Simplify your pattern or use more specific character classes.
Can I use this for programming?
Yes! The patterns you create here work directly in JavaScript. Most patterns also work in other languages with minor syntax adjustments.
Start Testing
Use the tool above to build and test your regular expressions. Start with a common pattern from our library or create your own from scratch.