According to the standards, extended regular expressions do not have back-references; only basic regular expressions have back-references. Therefore, with a standards conforming version of egrep (which the standards specify as grep -E (not egrep), it is almost impossible to find a variable string that appears twice on a line.

If you use grep instead of egrep (as Scrutinizer suggested in post #3), you can use it to print lines that have a string matching the basic regular expression (AKA BRE) pattern followed by a second occurrence of the same string.

The command:

Code:

grep '/.*|.*/'

and the command:

Code:

grep '/.*\|.*/'

will both print lines that contain a / immediately following by any string of 0 or more characters followed by a | followed by any string of 0 or more characters followed by a / (which does not seem to in any way match what you said you're looking for).

If you're looking for a string of one or more lower-case alphabetic characters (in a locale where the underlying codeset is a superset of ASCII) immediately followed by a by a duplicate of that same string (with nothing between them), you could get that using the grep command:

Code:

grep '\([a-z]+\)\1'

and if you wanted to find two adjacent words that appear at the start of a line or immediately follow a space and are followed by a space or the end of a line that occur next to each other separated by a single space, that would be something like:

Code:

grep -e '^\([a-z\) \1$' -e '^\([a-z\) \1 ' -e ' \([a-z\) \1 ' -e ' \([a-z\) \1$'

As noted by Scrutinizer in post #7, the above BREs are incorrect. The corrected form (assuming there is a single space character between words) is:

Code:

grep -e '^\([a-z][a-z]*\) \1$' -e '^\([a-z][a-z]*\) \1 ' -e ' \([a-z][a-z]*\) \1 ' -e ' \([a-z][a-z]*\) \1$'

In the above command the first BRE looks for two identical lower-case words alone on a line, the 2nd BRE looks for two identical words at the start of a line followed by one or more other words, the 3rd BRE looks for two identical words following one or more other words an followed by one or more other words, and the last BRE looks for two identical lower-case words at the end of a line following one or more other words.

Some versions of grep do not conform to the standards unless additional parameters are specified to force standards conformance. Without knowing what operating system you're using, we have no way of knowing if this problem might affect you.