I'm looking for the string "foo=" (without quotes) in text files in a directory tree. It's on a common Linux machine, I have bash shell:

grep -ircl "foo=" *

In the directories are also many binary files which match "foo=". As these results are not relevant and slow down the search, I want grep to skip searching these files (mostly JPEG and PNG images): how would I do that?

I know there are the --exclude=PATTERN and --include=PATTERN options, but what is the pattern format? manpage of grep says:

--include=PATTERN     Recurse in directories only searching file matching PATTERN.
--exclude=PATTERN     Recurse in directories skip file matching PATTERN.

Searching on grep include, grep include exclude, grep exclude and variants did not find anything relevant

If there's a better way of grepping only in certain files, I'm all for it; moving the offending files is not an option, I can't search only certain directories (the directory structure is a big mess, with everything everywhere). Also, I can't install anything, so I have to do with common tools (like grep or the suggested find).

UPDATES: @Adam Rosenfield's answer is just what I was looking for:

grep -ircl --exclude=*.{png,jpg} "foo=" *

@rmeador's answer is also a good solution:

grep -Ir --exclude="*\.svn*" "pattern" *

It searches recursively, ignores binary files, and doesn't look inside Subversion hidden folders.(...)