Question

I have an assignment in which I have to send to a file an unlimited list of parameters, the file will have to print the strings which are repeated in the following way:

NumNumNumCharCharChar…

• Num- number
• Char-character

every three following numbers are the same, as well as the three next characters, then another three numbers and then another three characters.
The string must start with numbers and end with characters in a repeated way.

In order to solve this question, you may use only grep/egrep — up to you, which means that the solution is in regular expressions..

OK, this is what I thought to do for the egrep:

egrep "^([0-9][0-9][0-9][a-b][a-b][a-b])\1*$"

Answer 1

Your attempt is almost correct. The backreference \1 will require repetitions of the matching string, not the matching pattern. Allow the pattern to repeat instead. Inside the repetitions, you do want backreferences:

egrep '^(([0-9])\2{2}([a-z])\3{2})+$' file

As a shell scripting tweak, I switched to single quotes (double quotes are less safe) and I extended the lowercase class to [a-z]. Note that the outer parentheses are group 1, so the backreferences to the inner parenthesized expressions will be \2 and \3.