What do $ ( ) and ^ expressions do in a event rule/monitor?

Hi,

I am trying to "reverse engineer" a rule that someone has previously written to show if accounts have been added into the domain admin or built in user groups. I can see it works and if I write itself myself with individual event ID's (632, 633, 636, 637) I know each work.

What they have done though is written it as one rule though -

"Matches regular expression" and the value is donw as -

^(632)(633)(636)(637)$

I just wondered is it documented what the ^ ( ) and $ are actually doing. It works I just want to know why..

thanks