What's the difference between () and [] in regular expression patterns?

Question

What is the difference between encasing part of a regular expression in () (parentheses) and doing it in [] (square brackets)?

How does this:

[a-z0-9] 

differ from this:

(a-z0-9) 

?

Answer 1

[] denotes a character class. () denotes a capturing group.

[a-z0-9] -- One character that is in the range of a-z OR 0-9

(a-z0-9) -- Explicit capture of a-z0-9. No ranges.

a -- Can be captured by [a-z0-9].

a-z0-9 -- Can be captured by (a-z0-9) and then can be referenced in a replacement and/or later in the expression.

Answer 2

(…) is a group that groups the contents like in math; (a-z0-9) is the grouped sequence of a-z0-9. Groups are particularly used with quantifiers that allow the preceding expression to be repeated as a whole: a*b* matches any number of a’s followed by any number of b’s, e.g. a, aaab, bbbbb, etc.; in contrast to that, (ab)* matches any number of ab’s, e.g. ab, abababab, etc.

[…] is a character class that describes the options for one single character; [a-z0-9] describes one single character that can be of the range a–z or 0–9.