Shell Programming and Scripting

Hi,

Please help me to understand the following code:

Code:

perl -lne 'print if "$_ " =~ /(5 (?:\d+ ){5})\1/'

What the regular expression "?:" does?
Also, whether the expression "\1" is the same as in sed (i.e) printing the elements inside pair of parentheses?

"(?: )" is the non-capturing (or grouping-only) parenthesis. It groups regex components, but does not capture them. And so, the regex components are not stored in variables $1, $2, $3, ... etc.

"\1" is the back-reference, or reference to an earlier parenthesized expression. Yes, its role in Perl is the same as in sed, but with certain limitations.

tyler_durden

Thanks durden_tyler !!

Can you also please explain what the command does?

Sure.
It prints the standard input if it is comprised of two groups of regex components, each of which consists of the digit 5 followed by a space followed by five groups of tokens, each of which, in turn, consists of one or more digits followed by a space.

tyler_durden