regexp greedness: shrinking long path - regex

Question

Please have a look at my mind-breaker.
I'd stuck in shrinking with regex some long path, like this:
/12345/123456/1234/123/12/1/1234567/13245678/123456789/1234567890
I'd like to transform this path to the following form:
/123/123/123/123/12/1/123/123/123/123
each "directory" in a path abbreviates to only 3 first characters
LONG_PATH="/12345/123456/1234/123/12/1/1234567/13245678/123456789/1234567890"
perl -pe "s#/(.{1,3})[^/]*?(/|$)#/\1\2#g" <<<$LONG_PATH
/123/123456/123/123/12//1234567/132/123456789/123
sed -E "s#/(.{1,3})[^/]*?(/|$)#/\1\2#g" <<<$LONG_PATH
/123/123456/123/123/12//1234567/132/123456789/123
I have tried also:
perl -pe "s,/(.)(.)?(.)?[^/]*+,/\1\2\3,g" <<<$LONG_PATH
/123/123/123/123/12//123/132/123/123
and many another, no "luck" - I still have no idea about.
Please point me a right way to success.

Answer 1

Match up to three non-slash characters and capture them. Then match the rest until the next slash. Replace by the capture:
"s#(/[^/]{3})[^/]*#\1#g"
There is no need for ungreediness or anything here, because the negated character class is mutually exclusive with the / or $.
EDIT: Although you seem to know this I should probably clarify for future visitors that this will work with either perl -pe... or sed -E... as you have used it in your question. The regex could also be used as is with sed -r.... If you leave out the -E or -r option, then (as usual) you will need to escape both the parentheses and curly brackets:
sed "s#\(/[^/]\{3\}\)[^/]*#\1#g" filename
Note also as ikegami points out that in Perl you should rather use $1 in the replacement than \1.

Answer 2

You could do it like this:
perl -pe's#[^/]{3}\K[^/]*##g'
/12345/123456/1234/123/12/1/1234567/13245678/123456789/1234567890
/123/123/123/123/12/1/123/132/123/123
Find 3 non-slashes, and keep (\K) them, remove the following characters up until the next slash.
As ikegami pointed out, it is not required to match less than three characters, in which case a lookbehind assertion can be used instead of \K. The benefit is that \K requires perl v5.10, and I believe look-around assertions predate that.
perl -pe 's#(?<=[^/]{3})[^/]*##g'

Answer 3

The best way seems to use the File::Spec module to split and recombine a path. An intermediate call to map will reduce each path segment to its first three characters. This program demonstrates
use strict;
use warnings;
use File::Spec;
my $path = '/12345/123456/1234/123/12/1/1234567/13245678/123456789/1234567890';
my $newpath = File::Spec->catdir(map substr($_, 0, 3), File::Spec->splitdir($path));
print $newpath;
output
/123/123/123/123/12/1/123/132/123/123