Question 1

What is a regular expression (regex)?

Accepted Answer

A regular expression (regex) is a sequence of characters that defines a search pattern. It is used to find, match, or replace text strings based on rules rather than exact values. For example, the pattern \d+ matches any sequence of one or more digits. Regex is supported in nearly every programming language.

Question 2

How do I test a regex pattern online?

Accepted Answer

Enter your regex pattern in the Pattern field and paste the text you want to test in the Test String area. Matches are highlighted in real time. You can set flags (g, i, m) and switch to Replace mode to see substitution results without writing any code.

Question 3

What regex flags are available in JavaScript?

Accepted Answer

The most common JavaScript regex flags are: g (global — find all matches, not just the first), i (case-insensitive matching), m (multiline — ^ and $ match start/end of each line), s (dotAll — dot matches newline), u (Unicode mode), and y (sticky — match only at the current position).

Question 4

What is the difference between greedy and lazy matching?

Accepted Answer

Greedy quantifiers (*, +, {n,m}) match as many characters as possible while still allowing the overall pattern to succeed. Lazy quantifiers (*?, +?, {n,m}?) match as few characters as possible. For example, against 'bold', the greedy <.*> matches the entire string, while the lazy <.*?> matches only ''.

Question 5

How do I match a whole word in regex?

Accepted Answer

Use word boundary anchors \b on both sides of the word. For example, \bcat\b matches 'cat' in 'the cat sat' but not in 'concatenate'. \b matches the position between a word character (\w) and a non-word character, ensuring the pattern is not part of a longer word.

\d	Digit [0-9]
\w	Word char [a-zA-Z0-9_]
\s	Whitespace
.	Any char (except \n)
[abc]	a, b, or c
[^abc]	Not a, b, or c

+	1 or more
*	0 or more
?	0 or 1
{n,m}	n to m times
^	Start of line
$	End of line

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