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Let's say:
/(a|b)/ vs /[ab]/
726
[] denotes a character class. () denotes a capturing group. [a-z0-9] -- One character that is in the range of a-z OR 0-9.
By placing part of a regular expression inside round brackets or parentheses, you can group that part of the regular expression together. This allows you to apply a quantifier to the entire group or to restrict alternation to part of the regex.
An escape can be either enclosing the phrase in braces, or placing a backslash before the escaped character. To pass a left bracket to the regular expression parser to evaluate as a range of characters takes 1 escape.
The special characters are: a. ., *, [, and \ (period, asterisk, left square bracket, and backslash, respectively), which are always special, except when they appear within square brackets ([]; see 1.4 below). c. $ (dollar sign), which is special at the end of an entire RE (see 4.2 below).
There's not much difference in your above example (in most languages). The major difference is that the () version creates a group that can be backreferenced by \1 in the match (or, sometimes, $1). The [] version doesn't do this.
Also,
/(ab|cd)/ # matches 'ab' or 'cd'
/[abcd]/ # matches 'a', 'b', 'c' or 'd'
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