What is a Regular Expression?

A regular expression (regex or regexp) is a sequence of characters that defines a search pattern. Regex patterns can match simple strings, complex character sequences, and structured formats like email addresses or URLs. They are supported natively in virtually every programming language and are a standard tool for text processing, validation, and data extraction.

A regex is written as a pattern enclosed in delimiters — for example /\d+/g — where the part between slashes is the pattern and the trailing letters are flags that modify matching behavior.

How Regex Works

The regex engine scans a string character by character and attempts to match the pattern at each position. Special syntax controls how matching behaves:

• Character classes like \d, \w, [a-z] match sets of characters.
• Quantifiers like *, +, {n,m} control how many times a token is matched.
• Anchors like ^ and $ assert a position rather than matching a character.
• Capture groups wrapped in () extract sub-matches and enable back-references in replacements via $1, $2, etc.
• Flags like g (global), i (case-insensitive), and m (multiline) change engine behavior.

How to Use This Tool

• Enter a pattern in the regex field — matches update live as you type.
• Toggle flags with the clickable badge buttons next to the pattern field.
• Load a common pattern from the Patterns dropdown to quickly test email, URL, IPv4, date, and other common formats.
• Click a match in the Matches panel to expand its start/end indices and capture group values.
• Enable Replace Mode to preview the result of a substitution using $1 back-references.
• Switch the Code tab between JS, Python, and Go to generate a ready-to-use snippet.
• Open the Cheat Sheet for a quick reference of syntax — click any item to insert it into the pattern field.
• Share the current regex, flags, and test string via a URL hash link.

Common Use Cases

• Validating user input — [a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,} for email addresses.
• Extracting structured data from logs — matching timestamps, error codes, or IP addresses with capture groups.
• Find-and-replace in editors like VS Code and JetBrains IDEs, both of which use $1 back-reference syntax.
• Parsing version strings using a pattern like (\d+)\.(\d+)\.(\d+) to capture major, minor, and patch components.
• Stripping HTML tags from a string using <[^>]+> with the global flag.
• Tokenizing source code or config files during build tooling or linting pipelines.