Question 1

What is a regular expression (regex)?

Accepted Answer

A regular expression is a sequence of characters that defines a search pattern. It's used for pattern matching within strings, validation, search and replace operations, and text parsing.

Question 2

What do the regex flags mean?

Accepted Answer

g (global) finds all matches, i makes matching case-insensitive, m enables multiline mode where ^ and $ match line boundaries, s allows . to match newlines, and u enables full Unicode support.

Question 3

What does \d mean in regex?

Accepted Answer

\d matches any digit (0-9). Similarly, \w matches word characters (a-z, A-Z, 0-9, _), \s matches whitespace, and their uppercase versions (\D, \W, \S) match the opposite.

Question 4

What's the difference between * and +?

Accepted Answer

* matches zero or more occurrences of the preceding element, while + matches one or more. For example, 'a*' matches '', 'a', 'aa', but 'a+' requires at least one 'a'.

Question 5

How do I match a literal special character?

Accepted Answer

Escape it with a backslash. For example, to match a literal dot, use \. instead of just . (which matches any character). Same for *, +, ?, etc.

Question 6

What are capture groups?

Accepted Answer

Parentheses () create capture groups that 'capture' the matched text for later use. Named groups use (?<name>pattern) syntax. Groups are useful for extracting parts of a match.

Pattern	Matches
`.`	Any character (except newline)
`\d`	Digit [0-9]
`\D`	Non-digit
`\w`	Word character [a-zA-Z0-9_]
`\W`	Non-word character
`\s`	Whitespace
`\S`	Non-whitespace
`[abc]`	Any of a, b, or c
`[^abc]`	Not a, b, or c
`[a-z]`	Character range

Pattern	Matches
`*`	0 or more
`+`	1 or more
`?`	0 or 1 (optional)
`{3}`	Exactly 3
`{3,}`	3 or more
`{3,5}`	Between 3 and 5

Pattern	Matches
`^`	Start of string/line
`$`	End of string/line
`\b`	Word boundary
`\B`	Non-word boundary

Pattern	Description
`(abc)`	Capture group
`(?:abc)`	Non-capture group
`(?<name>abc)`	Named group
`\1`	Back-reference to group 1
`a	b`

Flag	Name	Description
`g`	Global	Find all matches, not just the first
`i`	Case-insensitive	Ignore case differences
`m`	Multiline	^ and $ match line boundaries
`s`	Dotall	`.` matches newline characters
`u`	Unicode	Enable full Unicode matching

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