What is a Regular Expression (Regex)?
A regular expression is a pattern used to match, search, and manipulate text based on rules rather than exact strings.
A regular expression (regex) is a sequence of characters that defines a search pattern for text. Instead of matching exact strings, you describe rules — like "one or more digits" or "an email-shaped string" — and the regex engine finds matches.
How It Works:
- You write a pattern using special syntax
- A regex engine scans text looking for matches
- It returns matches, positions, or lets you replace them
Common Building Blocks:
.— any character*— zero or more of the previous+— one or more\d— a digit,\w— a word character[abc]— any of a, b, or c^and$— start and end of a line( )— capture groups
Where It's Used:
- Validation: Emails, phone numbers, passwords
- Search & replace: In editors and scripts
- Parsing: Extracting data from text
- Log analysis: Finding patterns in output
FAQ
Why do regular expressions look so cryptic?
They pack a lot of meaning into few characters. Once you learn the core symbols, patterns become readable — but complex ones are still worth commenting for clarity.
Are regular expressions the right tool for parsing HTML?
Usually not. HTML is nested and irregular, which regex handles poorly. Use a proper parser for structured formats like HTML, XML, or JSON.