Regex for specific problem -

I use this stupid newby problem to regex and even not enabled (maybe too tired?) I'm not familiar with:

I need a regex, which allows any combination of numbers, letters (lower and upper case) and underscore _

But: The beginning of this regex will be defined and defined by my source code ::

  ABC_h2u3h4l ABCijij4i5oi4j5 ABCABBC  

Here's the piece "Abyss Always " should always be in the main place.

Can someone give me a hint?

  ^ ABC [AZA-Z0-9 _] * $