Grepping Specific Letter Instances

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http://www.unix.com – Hi! I'm trying to teach myself regex by answering word puzzle questions on "answer" sites, seeing if I can form a regex that'll grep the word(s) they're looking for. I do ok when the question is specific - a thirteen-letter word ending in "tion" for instance, or where they know the word length and letter positions - but words containing certain letters, or words containing a number of instances of one letter, have me stumped. For instance, I cannot figure out how to find words containing four e's. I suspect it's a syntax problem. Many web sites will tell you what each symbo (HowTos)