Question 1

What is a regular expression?

Accepted Answer

A regular expression (regex) is a sequence of characters that defines a search pattern. They're used for string matching, validation, search-and-replace and parsing in nearly every programming language and text editor.

Question 2

What's the difference between greedy and lazy quantifiers?

Accepted Answer

Greedy quantifiers (like *, +, ?) match as much text as possible. Lazy quantifiers (*?, +?, ??) match as little as possible. For example, on 'tag', the pattern '<.*>' is greedy and matches the whole string, while '<.*?>' is lazy and matches just ''.

Question 3

What is a lookahead in regex?

Accepted Answer

A lookahead (?=pattern) checks that what follows matches the pattern without including it in the match. Negative lookahead (?!pattern) checks that what follows does NOT match. For example, 'q(?=u)' matches 'q' only when followed by 'u'.

Question 4

Are regex patterns the same across languages?

Accepted Answer

Most basic regex features are universal. However, advanced features like lookbehind, atomic groups and named groups may have differences. JavaScript, Python, Java, PCRE (PHP) and .NET each have their own extensions.

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