How to grep for repeated patterns of repeated patterns?

Question

I have a text file, fred.txt:

% cat -e fred.txt 
00:$
00:04:$
01:00:23:34$
01:$
01:40:$
01:40:32:$
%

I can grep for a line with 2 digits and a colon:

% pcregrep -e '[\d]{2}:' fred.txt   
00:
00:04:
01:00:23:34
01:
01:40:
01:40:32:
%

but when I try to get repeating patterns of that pattern, it doesn't find them:

% pcregrep -e '[[\d]{2}:]{2}' fred.txt
%

I'm looking to get the same output as this:

% pcregrep -e '[\d]{2}:[\d]{2}:' fred.txt
00:04:
01:00:23:34
01:40:
01:40:32:
%

Eventually I'll be looking for more nested repeating patterns in a larger file so I don't want to define each time the pattern repeats. How do I grep for the lines that have that pattern repeating?

Answer 1

Using GNU grep

$ grep -Eo '([0-9]{2}:){2,}' fred.txt 
00:04:
01:00:23:
01:40:
01:40:32:

Answer 2

[xyz] called bracket expressions is to match one character within a set. Here, either x, y or z.

[\d] would match either \ or d in POSIX basic regexps, but in perl-compatible regexps, it matches the same as \d alone: a digit. That allows things like [\da-fA-F] to match one hex digit for instance, but in the [\d] case, it's pointless, as you might as well use \d.

In any case, it's not for grouping for which you need \(...\) in POSIX basic regexps or (...) in POSIX extended regexps or PCREs.

PCREs have a few other variants of grouping such as (?:...) which groups without creating a back-reference, or (?|...) which groups but affects how back-references are numbered in alternations, or (?>...) the possessive variant of the grouping operator, etc.

Here, you don't need backreferences, so you might as well use (?:...):

pcregrep '(?:\d{2}:){2}'

Would match on lines that contain 2 of (2 digits followed by a :). If you wanted to match on lines that are exactly that, you'd need to add the -x option.

Here,

pcregrep '(?:\d\d:){2}'

Would actually be shorter.

pcregrep '(\d\d:){2}'

Would have the same effect, but possibly be slightly less efficient as we didn't tell pcregrep not to bother capturing what's inside the (...).

See man pcrepattern to learn more about the syntax of PCREs.

Answer 3

Using grep

$ grep -Eo '([^:]*:[[:digit:]]+:?)+' input_file
00:04:
01:00:23:34
01:40:
01:40:32: