CODEWITHSUNDEEP
bashsed
mahmood
mahmood

Reputation: 24795

replacing a string using SED doesn't work

Consider this line:

--dump-config=h264_sss-l2-2-ghb-16-500.ini --stats-file=h264_sss-l2-2-ghb-16-500.stat configs/example/cmp.py --l2cache -b h264_sss

and this string "l2-2-ghb-16". To change that string with SED, I ran this command:

sed  's/l2-.*-.*-.*-/l2-2-ghb-8-m-/g'

But the whole line then changed to 

--dump-config=h264_sss-l2-2-ghb-8-m-b h264_sss

What is the problem

Upvotes: 1

Views: 173

Answers (3)

paxdiablo
paxdiablo

Reputation: 882596

That regex is greedy inasmuch as .* will match the maximum number of characters.

That means it will attempt to stretch the match from what you think is the first pattern all the way to what you think is the second.

While you may think there are two matched patterns, the fact that this stretching is happening means that there's only one, and it's longer than you think.

A quick fix is to ensure that it doesn't match beyond the next - character with something like:

sed  's/l2-[^-]*-[^-]*-[^-]*-/l2-2-ghb-8-m-/g'

as per the following transcript:

pax> echo '--dump-config=h264_sss-l2-2-ghb-16-500.ini  --stats-file=h264_sss-l2-2-ghb-16-500.stat  configs/example/cmp.py --l2cache -b h264_sss' | sed 's/l2-[^-]*-[^-]*-[^-]*-/l2-2-ghb-8-m-/g'
           --dump-config=h264_sss-l2-2-ghb-8-m-500.ini --stats-file=h264_sss-l2-2-ghb-8-m-500.stat configs/example/cmp.py --l2cache -b h264_sss

(command and output are slightly modified, lined up so you can easily see the transformations).

This works because, while .* says the largest field of any characters, [^-]* says the largest field of any characters except -.

Upvotes: 2

mykel
mykel

Reputation: 426

The .* portion matches the longest possible stretch of characters it can to make the pattern work. So the first .* doesn't match just 2 as you hope, but 2-ghb-16-500.ini --stats-file=h264_sss-l2-2-ghb-16, and so on. To make it work replace the dot's with [^-] (any non-dash character). So,

sed  's/l2-[^-]*-[^-]*-[^-]*-/l2-2-ghb-8-m-/g'

Upvotes: 4

ezdazuzena
ezdazuzena

Reputation: 6790

sed looks for the most possible match. So -.*- will match a string as large as possible.

Upvotes: 0

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