m4_pattern_forbid

[Matěj Týč]

Hello,
I am doing some m4sugar programming and I found the m4_pattern_forbid 
insufficient.
As we all know, there are many kinds of regular expression in different 
programs and even on different OSs (grep, vim, Python, linux sed, osx 
sed, ...). So my question is - where should I look to learn what kind of 
regular expression I can specify to that macro?

Interestingly, I took a look at the autoconf/m4sugar/m4sugar.m4:2005 to 
learn the definition and I was surprised by what I have found there:

# m4_pattern_forbid(ERE, [WHY])
# -----------------------------
# Declare that no token matching the forbidden extended regular
# expression ERE should be seen in the output unless...
m4_define([m4_pattern_forbid], [])

I was unable to find the true macro's definition anywhere, so here is 
the second question: How (and where) is m4_pattern_forbid defined?