regex - 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.