Question 1

What is a regular expression?

Accepted Answer

A regular expression (regex) is a sequence of characters that defines a search pattern. It is used for string matching, validation, parsing, and substitution in text. Regular expressions are supported in virtually every programming language and many text editors.

Question 2

What does the global flag (g) do in a regex?

Accepted Answer

The global flag (g) makes the regex find all matches in the input string, rather than stopping after the first match. Without the g flag, only the first occurrence is returned. Use g when you want to find or replace every instance of a pattern.

Question 3

What is a capture group in regex?

Accepted Answer

A capture group is a portion of a regex pattern enclosed in parentheses, e.g. (\d+). The text matched by a capture group is captured and available as a numbered reference ($1, $2…) or named reference (?<name>). Capture groups are useful for extracting parts of a match or for backreferences.

Question 4

What is the difference between the i and m flags?

Accepted Answer

The i flag makes the regex case-insensitive, so it matches both upper and lower case letters without explicitly listing them. The m flag (multiline) changes the behavior of ^ and $ anchors: without m, they match the start and end of the entire string; with m, they match the start and end of each line.

Question 5

How do I match a literal dot or other special characters?

Accepted Answer

In regex, many characters have special meaning (. * + ? [ ] { } ( ) ^ $ | \). To match a literal version of any of these characters, escape it with a backslash. For example, \. matches a literal dot, \* matches a literal asterisk, and \ matches a literal backslash.

.	Any char (except newline)
\d	Digit [0-9]
\w	Word char [a-zA-Z0-9_]
\s	Whitespace
^	Start of string/line
$	End of string/line
*	0 or more
+	1 or more
?	0 or 1
{n,m}	Between n and m
(…)	Capture group
(?:…)	Non-capture group
[abc]	Character class
[^abc]	Negated class
a\|b	Alternation

Flag	Name	Description
g	Global	Find all matches, not just the first one.
i	Case-insensitive	Match upper and lower case letters equally.
m	Multiline	^ and $ match start/end of each line, not just the full string.
s	Dot-all	The . metacharacter also matches newline characters.
u	Unicode	Enable full Unicode matching (e.g. for emoji and surrogate pairs).

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