Regex to match when a string is present twice

Question

I am horrible at RegEx expressions and I just don\'t use them often enough for me to remember the syntax between uses.

I am using grepWin to search my files. I need

Answer 1

This is significantly harder than I originally thought it would be, and requires variable-length lookbehind, which grepWin does not support...

this expression:

 (?

was successfully used in Eclipse, using the "Search > File" dialog to exclude files with one and three instances of blah and to include files with exactly two instances of blah.

Eclipse does not permit a .* in lookbehind, so I used .{0,99999} instead.

It is possible, with the right tool, but It isn't pretty to get it to work with grepWin (see answer above). Can you use other tools (such as Eclipse) and what did you want to do with the files afterwards?