What Are Regular Expressions?
Regular expressions (regex) are patterns used to match character combinations in strings. They are supported in virtually every programming language and are essential for text processing, input validation, search-and-replace, and log parsing.
JavaScript Regex Flags
- g — Global: find all matches, not just the first
- i — Case-insensitive matching
- m — Multiline: ^ and $ match line boundaries
- s — Dotall: . matches newline characters
Common Use Cases
- Validating email addresses, phone numbers, and URLs
- Extracting data from log files and structured text
- Search-and-replace in code editors and IDEs
- Building input masks and form validators
- Parsing and transforming CSV, JSON, and XML data
When To Reach For Regex
Regular expressions shine for short, well-defined patterns where you need to match, extract, or replace text — log line parsing, URL routing rules, form-field validation, and search-and-replace inside an editor. They become a liability when the input is actually a structured language: HTML, JSON, source code, or anything with nested delimiters. For those, use a real parser; the regex you write today will silently break on the next edge case.