Question 1

What is a regular expression (regex)?

Accepted Answer

A regular expression (regex or regexp) is a sequence of characters that defines a search pattern. They are used in programming, text editors, and command-line tools to find, match, replace, or validate strings. For example, \d+ matches one or more digits.

Question 2

What are regex flags?

Accepted Answer

Flags modify how a regex pattern is applied. Common flags: 'g' (global — find all matches, not just the first), 'i' (case-insensitive), 'm' (multiline — ^ and $ match line starts/ends), 's' (dotAll — . matches newlines), 'u' (Unicode support). Combine flags like 'gi' for global case-insensitive matching.

Question 3

How do I match an email address with regex?

Accepted Answer

A common email regex pattern is: [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,} — This matches most standard email formats. However, the official email specification (RFC 5322) is extremely complex. For production use, combine a simple regex with server-side validation.

Question 4

What is the difference between * and + in regex?

Accepted Answer

The * quantifier matches zero or more occurrences of the previous character/group, while + matches one or more. For example, 'ab*c' matches 'ac', 'abc', 'abbc', etc. But 'ab+c' requires at least one 'b', so it matches 'abc', 'abbc', but not 'ac'.

Question 5

How do I escape special characters in regex?

Accepted Answer

Special regex characters (. * + ? ^ $ { } [ ] ( ) | \) need to be escaped with a backslash to match literally. For example, to match a period, use \. instead of just a dot. To match a backslash itself, use \.

Question 6

What are capture groups in regex?

Accepted Answer

Capture groups, created with parentheses (), let you extract specific parts of a match. For example, (\d{3})-(\d{4}) applied to '555-1234' captures '555' in group 1 and '1234' in group 2. Use (?:...) for non-capturing groups when you need grouping without capturing.

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