Regular Expressions (Regex)

Regular expressions define search patterns using special characters and quantifiers.

Common patterns

• ^ and $: Match start and end of string.
• . : Any character except newline.
• \d, \w, \s: Digits, word characters, whitespace.
• +, *, ?: One or more, zero or more, zero or one.
• [abc], [^abc]: Character sets and negation.

Use cases

• Validate email addresses, phone numbers, URLs.
• Extract data from logs and text files.
• Find and replace patterns in code.
• Input sanitization and data parsing.

Security warnings

• Catastrophic backtracking (ReDoS) can cause denial of service.
• Always test regex performance with adversarial inputs.