Regex Tester

Regular Expressions: A Complete Guide

Regular expressions (regex or regexp) are sequences of characters that define search patterns. They are one of the most powerful tools in a developer's toolkit for text processing, validation, and data extraction. Regex is supported in virtually every programming language including JavaScript, Python, Java, C#, Go, Ruby, PHP, and many more. Understanding regex can dramatically improve your ability to work with text data efficiently.

Regex Syntax Fundamentals

At its core, a regex pattern describes a set of strings. Literal characters match themselves, while special metacharacters provide matching logic. The most important metacharacters include: the dot (.) which matches any character except a newline; the caret (^) and dollar sign ($) which anchor matches to the start and end of a string; and quantifiers like * (zero or more), + (one or more), and ? (zero or one). Character classes ([abc]) match any single character in the set, while negated classes ([^abc]) match any character not in the set. Shorthand classes include \d for digits, \w for word characters, and \s for whitespace.

Common Regex Patterns

• Email validation: [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,} matches most standard email addresses.
• URL matching: Patterns for URLs typically start with https?:// followed by character classes for the domain, path, and query string components.
• Phone numbers: US phone patterns account for optional parentheses, dashes, dots, and spaces as separators.
• IP addresses: IPv4 addresses match four groups of one to three digits separated by dots.

Lookahead and Lookbehind Assertions

Lookahead ((?=...)) and lookbehind ((?<=...)) are zero-width assertions that match a position without consuming characters. Positive lookahead asserts that what follows the current position matches the pattern. Negative lookahead ((?!...)) asserts the opposite. These are invaluable for complex matching scenarios like password validation rules (must contain a digit AND a special character) or extracting text between specific delimiters without including the delimiters themselves.

Performance Considerations

Regex engines can be surprisingly slow with certain patterns, especially those involving nested quantifiers or excessive backtracking. Patterns like (a+)+ can cause catastrophic backtracking, where the engine explores an exponential number of paths before determining there is no match. To avoid this, use atomic groups or possessive quantifiers when available, prefer specific character classes over the dot, and avoid nesting quantifiers. Always test your regex against both matching and non-matching input to ensure acceptable performance.

Regex Across Different Languages

While the core regex syntax is similar across languages, there are important differences. JavaScript uses the RegExp object and supports flags like g, i, m, s, and u. Python's re module uses raw strings (prefix r) to avoid backslash conflicts. Java requires double-escaping backslashes in string literals. Some languages support features that others do not: for example, lookbehind must be fixed-width in JavaScript but can be variable-width in Python and .NET. Always consult the documentation for your specific language when using advanced regex features.