Your regular expression has only two small errors:
$regexp(%title%,^A |An |The |,)
should be - as @ohrenkino says:
$regexp(%title%,^(A |An |The ),)
which means:
The pipe symbol | gives you a selection:
A or An or The (all three followed by a space)
The pipe symbol should not be the last character.
The brackets around this selection represent the caption group, which will be replaced if one of the 3 possibilities starts ^ as the first character(s) of the line.
The matched possibility will be replaced with nothing (nothing is given after the comma).