Here's a little one-line command that will search all files in a directory with a given filename, and do a text substitution on them:
find . -name "*.html" -exec perl -pi -w -e 's/BADREGEX/GOODWORD\//g;' {} \;
In this case, I'm searching the current directory for all files with the '.html' extention. I'm then replacing all instances of 'BADREGEX' in the file with 'GOODWORD'.
If you pull the perl out of the command, you can use it on individual files:
perl -pi -w -e 's/BADREGEX/GOODWORD\//g;' [filename]