$str = 'blfoo and barthisfoothatqux and barsofooquxhim andfoosom foo';

# replace all foo(s) above with 123 except for the ones that are
# surrounded by bar and qux

# note: there are 5 foo(s) in $str above; only 3 should be replaced 
# so $str becomes
# 'bl123 and barthisfoothatqux and barsofooquxhim and123som 123'