What is Regex (Regular Expressions)?
Regular Expressions (regex) are sequences of characters that define search patterns for text matching, validation, and manipulation. Developed by mathematician Stephen Cole Kleene in the 1950s as part of automata theory, regex became a cornerstone of computational linguistics and text processing. Today, every major programming language (JavaScript, Python, Java, C#, Ruby) includes built-in regex support. Unlike simple text search which only finds exact matches, regex can describe flexible patterns like "any three digits followed by a dash and four more digits" (matching phone numbers) or "text between quotation marks."
The power of regex lies in its expressiveness. A single pattern can match thousands of variations: `\d{3}-\d{4}` matches all seven-digit phone numbers formatted as XXX-XXXX. This makes regex essential for form validation, data extraction from logs, search-and-replace operations, and parsing structured text formats. While regex syntax appears cryptic at first glance, learning the core metacharacters unlocks powerful text processing capabilities.
How Does a Regex Tester Work?
Our regex tester provides a visual interface to JavaScript's native RegExp engine. When you enter a pattern (e.g., `[A-Za-z]+`) and test text, the tool compiles your regex into a RegExp object, applies it to the test string, and highlights all matches in yellow. The tool supports three flags: global (`g`) finds all matches instead of stopping at the first, case-insensitive (`i`) ignores capitalization, and multiline (`m`) makes `^` and `$` match line boundaries instead of just string boundaries.
Real-time testing accelerates the learning curve. As you modify your pattern, the highlighted results update instantly, providing immediate feedback. If your regex contains syntax errors (like unmatched brackets or invalid escape sequences), the tool displays an error message explaining what went wrong. This trial-and-error process is how most developers master regex—by experimenting with patterns and observing results.
Common Regex Patterns and Uses
- Email validation: `[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Z|a-z]{2,}` matches most email formats
- Phone numbers: `\d{3}-\d{3}-\d{4}` matches US format like 555-123-4567
- URLs: `https?://[^\s]+` matches web addresses starting with http or https
- Dates: `\d{2}/\d{2}/\d{4}` matches MM/DD/YYYY format
- IP addresses: `\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}` matches IPv4 addresses
- Hex colors: `#[0-9A-Fa-f]{6}` matches CSS color codes like #FF5733
Regex Metacharacters Quick Reference
.— Matches any single character except newline\d— Matches any digit (0-9)\w— Matches word characters (letters, digits, underscore)\s— Matches whitespace (spaces, tabs, newlines)^— Anchors to start of string/line$— Anchors to end of string/line*— Matches 0 or more repetitions (greedy)+— Matches 1 or more repetitions (greedy)?— Matches 0 or 1 occurrence (makes quantifiers non-greedy when used after * or +)[abc]— Matches any character in brackets (a, b, or c)[^abc]— Matches any character NOT in brackets()— Creates capturing group for extraction
Frequently Asked Questions (FAQs)
What regex flavor is supported?
This tool uses JavaScript's native RegExp engine. It supports standard regex features including lookaheads, capturing groups, and flags.
How do flags work?
Flags modify how the search works. 'g' (global) finds all matches, 'i' (insensitive) ignores case, and 'm' (multiline) affects anchors like ^ and $.
Why is my regex matching too much?
Regex quantifiers (* and +) are greedy by default, matching as much as possible. To make them non-greedy, add a '?' after the quantifier (e.g., .*? instead of .*).
How do I match an email address?
A basic pattern is: [A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Z|a-z]{2,}. This matches most common email formats but may not cover all edge cases defined in RFC 5322.
What are capturing groups?
Capturing groups use parentheses to extract parts of a match. For example, (\d{3})-(\d{4}) matches phone numbers like 555-1234 and captures the two parts separately.
Can I test multiline regex patterns?
Yes, enable the 'm' (multiline) flag to make ^ and $ match the start/end of each line instead of just the start/end of the entire string.