Regex for specific problem

I'm not very familiar to regex and even not able (maybe too tired?) to use this silly newby issue:

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

BUT: The beginning of this regex shall be fix and defined in my source code::

ABC_h2u3h4l
ABCijij4i5oi4j5
ABCABC

Here the piece "ABC" always has to be at the leading posi开发者_JAVA技巧tion.

Can someone give me a hint?

that's the whole regex:

^ABC\w+

Something like this?

^ABC\w+$

^ABC[a-zA-Z0-9_]*$