Question 1

What is Regex?

Accepted Answer

Regular expressions (regex) are sequences of characters that define search patterns. They are used for string searching, validation, extraction, and replacement in virtually every programming language. A regex like \d+ matches one or more digits anywhere in a string.

Question 2

How do regex capture groups work?

Accepted Answer

Capture groups are created by wrapping part of a pattern in parentheses: (\w+). When the regex matches, the content inside the group is captured separately. Group 1 is $1 or \1 in replacements, Group 2 is $2, and so on. Named groups use (?<name>pattern) syntax.

Question 3

What are regex flags?

Accepted Answer

Flags modify how a regex pattern behaves. Common flags: g (global — find all matches), i (case insensitive), m (multiline — ^ and $ match line boundaries), s (dotAll — dot matches newlines), u (unicode — enables full Unicode matching), y (sticky — match from lastIndex only).

Question 4

How do I validate email addresses with regex?

Accepted Answer

A common email pattern is: [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}. This matches most standard email formats. For production use, always validate emails server-side or via a dedicated library, as truly complete email regex is extremely complex.

Question 5

How do I extract URLs from text?

Accepted Answer

Use: https?:\/\/[^\s/$.?#].[^\s]* with the global flag. This matches http:// and https:// URLs. For more robust URL extraction, add word boundary anchors and handle edge cases like trailing punctuation.

Question 6

How do replacements work in regex?

Accepted Answer

In the replacement field, $1 refers to capture group 1, $2 to group 2, and $& to the full match. For example, pattern (\d+) with replacement [$1] wraps every number in brackets. The global flag determines whether one or all matches are replaced.

Question 7

What is greedy vs lazy matching?

Accepted Answer

Greedy quantifiers (*, +, {n,m}) match as much as possible. Lazy quantifiers (*?, +?, {n,m}?) match as little as possible. Example: <.+> on 'bold' greedily matches the whole string; <.+?> lazily matches just ''.

Question 8

What is the difference between ^ and \b?

Accepted Answer

^ anchors to the start of the string (or line with the m flag). \b is a word boundary — it matches the position between a word character (\w) and a non-word character. Use ^ for line/string start anchoring and \b for matching whole words.

Flag	Name	Effect
g	Global	Find all matches
i	Ignore Case	Case insensitive
m	Multiline	^ $ match line ends
s	Dot All	Dot matches newline
u	Unicode	Full Unicode support
y	Sticky	Match from lastIndex

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