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).
Thank You for explaining It. Trying to learn off other peoples posts is hard a lot of the time. The end results of what it should look like are not provided. I am one of those people that has to see it to understand it, especially if I am unfamiliar with the context of what is happening.