You unfortunately will disallow strings with bar *after* the second foo. What you want is this: 
/foo((?:(?!bar).)*)foo/

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Note that (?:stuff) is a non-grouping set of parens, and (?!stuff) is a negative assertion.

UPDATE
I noticed a while ago that my RE was incorrect, decided to leave it for someone else to find and mention. But nobody did so I will mention that I forgot a ?: 

/foo((?:(?!bar).)*?)foo/

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In reply to RE (tilly) 2: Regex'ing backreferences by tilly
in thread Regex'ing backreferences by Uruk

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