Need help with Grep command

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http://www.unix.com – I've just recently started using a Unix server, and I've been trying to figure out how to use the grep command to search a file of mine for all 4-letter words that begin with 's' (I don't care if its uppercase or lowercase) and have an 'r' as the third letter. I've tried many grep variations but none of them have returned properly with what I need. All help is appreciated, thanks. (HowTos)