Grep lines that match pattern twice

Question

Usually grep outputs any lines that match a pattern. I want to be able to find lines that match the pattern multiple times. For example, if my search pattern was "foo", then:

foo bar      # Would not be matched
foo foo bar  # Would be matched
bar foofoo   # Would be matched
foobarfoo    # Would be matched

Is there a way I can tell grep to find only lines that contain multiple matches of my search pattern?

Answer 1

grep -E "(foo.*){2}" file

This matches at least 2 times on each line of file or output, you can give minimum number of matches.

Answer 2

If you want to match all lines that matches any string twice:

grep '\(.\{1,\}\).*\1'

You can change the length to match by changing 1,:

seq 10000 | grep '\(.\{2,\}\).*\1'

This uses Basic Regular Expression (BRE) and should therefore work on any POSIX compliant grep.

If you convert the regexp to use non-greedy regexp (not supported everywhere) it does not seem to speed up matching:

grep -E '(..*?).*?\1'

The graph shows runtime in seconds of runs with and without non-greedy on 100 lines of each n numbers (~ line length).

greedy() {
  a=`seq $1`;
  yes $a | head -n 100 | grep '\(.\{1,\}\).*\1' | LC_ALL=C wc;
}
nongreedy() {
  a=`seq $1`;
  yes $a | head -n 100 | grep -E '(..*?).*?\1' | LC_ALL=C wc;
}
export -f greedy
export -f nongreedy
parallel --jl my.log {2} {1}000 {2} ::: {1..100} ::: greedy nongreedy

Answer 3

grep "foo.*foo" file.txt

That will only return lines where foo appears two or more times. It won't return lines where it only appears once.

The above will work without quotes most of the time but in other cases where there are filenames in the directory that match the glob such as foo.barfoo, for example, it is necessary to quote the regex which is why I have edited my answer to include double quotes.