Using alternation and grouping constructs

Question

Originally I wanted a regex parsing month numbers. At first I came up with the following regex:

^([1-9]{1})|(1[012])$

and it matched any positi

Answer 1

The alternation operator has the lowest precedence of all regex operators.

The difference between the two regular expressions, interpreted literally, is this:

( [BEGIN]([1-9]) )    OR    ( (1[012])[END] )

vs

[BEGIN] ( [1-9]    OR    1[012] ) [END]