gawk 3.1.3 vs gawk 3.1.1
 

Using gawk 3.1.3 in Linux
echo 'This should match abc_123_abc' | gawk '/ab\w+bc/ {print "It matches"}'

returns "It matches"

Using gawk 3.1.1 in HPUX
echo 'This should match abc_123_abc' | gawk '/ab\w+bc/ {print "It matches"}'

returns nothing.

I don't know if this is a version thing or an architecture thing.

Any ideas?

How about something simple like:
If it does I guess you need to build your regex by pieces to see what is broken.
\w appears as a standard piece of awk and its variants.

This is solved by moving to version 3.1.7.