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

I don't know what grepWin supports, but here's what I came up with to make something match exactly twice.

/^((?!how).)*how((?!how).)*how((?!how).)*$/

Explanation:

/^             # start of subject
  ((?!how).)*  # any text that does not contain "how"
  how          # the word "how"
  ((?!how).)*  # any text that does not contain "how"
  how          # the word "how"
  ((?!how).)*  # any text that does not contain "how"
$/             # end of subject

This ensures that you find two "how"s, but the texts between the "how"s and to either side of them do not contain "how".

Of course, you can substitute any string for "how" in the expression.

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If you want to "simplify" by only writing the search expression twice, you can use backreferences thus:

/^(?:(?!how).)*(how)(?:(?!\1).)*\1(?:(?!\1).)*$/

Refiddle with this expression

Explanation:
I added ?: to make the negative lookaheads' text non-capturing. Then I added parentheses around the regular how to make that a capturing subpattern (the first and only one).

I had to include "how" again in the first lookahead because it's a negative lookahead (meaning any capture would not contain "how") and the captured "how" is not captured yet at that point.