Question 1

What does . (dot) match in regex?

Accepted Answer

By default, . (dot) matches any single character except a newline. With the dotAll (s) flag enabled, . also matches newlines. To match a literal dot, escape it: \.

Question 2

What is the difference between * and +?

Accepted Answer

* (star) matches zero or more occurrences of the preceding element. + (plus) matches one or more occurrences. For example, a* matches '', 'a', 'aa'; while a+ requires at least one 'a' and does not match an empty string.

Question 3

What are anchors in regex (^ and $)?

Accepted Answer

^ matches the start of the string (or line in multiline mode). $ matches the end of the string (or line in multiline mode). For example, ^hello$ matches only the exact string 'hello' with nothing before or after it.

Question 4

What is a non-greedy quantifier?

Accepted Answer

By default, quantifiers like * and + are greedy — they match as much as possible. Adding ? makes them non-greedy (lazy): they match as little as possible. For example, <.*> matches 'bold' entirely, while <.*?> matches only ''.

Question 5

What are lookaheads and lookbehinds?

Accepted Answer

Lookaheads (?=...) and (?!...) assert what follows a match without including it. Lookbehinds (?<=...) and (?<!...) assert what precedes a match. For example, \d+(?= dollars) matches a number only when followed by ' dollars'.

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