A regular expression (regex) is a powerful pattern-matching tool used to search, match, and manipulate text. Regular expressions provide a concise way to describe complex text patterns, making them essential for text processing, validation, data extraction, and search operations. Whether you're validating email addresses, extracting data from logs, or searching for specific patterns in code, regex is an indispensable tool for developers.

Regular expressions work by defining a pattern that describes a set of strings. When applied to text, the regex engine searches for matches and can extract specific parts using capture groups. For example, the pattern \b\w+@\w+\.\w+\b matches email addresses, while \d{3}-\d{3}-\d{4} matches phone numbers in the format XXX-XXX-XXXX.

Here are some common regex patterns that developers use frequently:

• Email validation: \b\w+@\w+\.\w+\b - Matches basic email addresses
• Phone numbers: \d{3}-\d{3}-\d{4} - Matches XXX-XXX-XXXX format
• URLs: https?://[^\s]+ - Matches HTTP/HTTPS URLs
• IP addresses: \d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3} - Matches IPv4 addresses
• Dates: \d{4}-\d{2}-\d{2} - Matches YYYY-MM-DD format
• Credit cards: \d{4}[\s-]?\d{4}[\s-]?\d{4}[\s-]?\d{4} - Matches card numbers with optional separators
• Hex colors: #[0-9A-Fa-f]{6} - Matches hexadecimal color codes

These patterns can be customized for your specific needs. For example, when working with JSON data that contains URLs, you can use our JSON Viewer tool to inspect the structure, then use regex patterns to extract specific values. Similarly, when processing URLs, our URL Encode tool can help encode special characters that might need escaping in regex patterns.

Regex flags modify how a pattern behaves during matching. Our regex tester supports all standard JavaScript flags:

• g (global): Find all matches instead of stopping after the first match. Essential when you need to find multiple occurrences in a string.
• i (case insensitive): Makes the pattern match regardless of case. Useful for user input validation where case shouldn't matter.
• m (multiline): Makes ^ and $ match the start and end of each line, not just the start and end of the entire string. Crucial for processing multi-line text.
• s (dotall): Makes the . character match newline characters as well. Useful when you need to match across line breaks.

Flags can be combined. For example, gi enables both global matching and case-insensitive matching. Understanding flags is crucial for writing effective regex patterns that work correctly in different scenarios.

Regular expressions can be powerful but also dangerous if not used carefully. Here are common performance pitfalls and how to avoid them:

Capture groups allow you to extract specific parts of a match. Groups are created using parentheses and are numbered starting from 1 (group 0 is the entire match). For example, in the pattern (\w+)@(\w+\.\w+), group 1 captures the username and group 2 captures the domain.

Our regex tester displays all capture groups for each match, making it easy to see what each group captured. This is particularly useful when extracting structured data from text, validating input formats, or parsing log files. When working with structured data like JSON, you can combine regex with our JSON Viewer tool to extract and validate specific fields.

All regex testing happens entirely in your browser using JavaScript. Your patterns and test strings never leave your device, are never sent to any server, and are never stored. This privacy-first approach ensures that sensitive data like log files, user input, or confidential text remains completely private and secure. Whether you're testing regex patterns for production code, debugging text processing issues, or learning regex syntax, your data stays local and secure.