CODEWITHSUNDEEP
regexsedsh
sflee
sflee

Reputation: 1719

Regular expression, sed, path extraction


This is a question about regular expression.
I have the following code:

Program

#!/bin/sh

temp="/home/user/game/log.txt"
echo $temp | sed -e "s#\(.*\)/.*#\1#"

Output

/home/user/game

Question

What is the meaning of this guy --- s#\(.*\)/.*#\1#?

I search from the web and I know some meaning, for example:

s - substitute

. - any word

* - the word in front of this can repeat 0 to infinite times

But I still don't understand the meaning of this code,although I know the function

Upvotes: 0

Views: 2711

Answers (2)

perreal
perreal

Reputation: 98088

"s#\(.*\)/.*#\1#"

s is the substitute command, its format is like s[DELIM]regex[DELIM]replace[DELIM]. and DELIM can be any character delimiting the arguments. In your case, the delimiter is #. In the regex part you have:

\(.*\)/.*

Here the match between \( and ) is captured, meaning that you can refer to it in the replace part of the command. Inside the capture you are [greedy] matching anything. But this match has to stop somewhere because after the capture there is a slash / which should be matched. Since the captured .* is greedy, sed will match and capture until the last slash. Then, it will match .* without capturing. This part won't contain any slashes (due to previous greedy match). Thus, the regex will match all input if it contains a slash, but it will also remember the part until the last slash.

the replace part:

\1

replaces the matched pattern with the captured part. All in all, this command matches files within some directory and will remove filename, leaving only the directory name.

Upvotes: 1

user1907906
user1907906

Reputation: 

 s#\(.*\)/.*#\1
  • s: substitue command
  • #: delimeter of the three parts the s command has
  • \(\): a group that can later be referenced
  • .: any character
  • /: a literal slash
  • .*: any number of characters
  • \1: referecen to the first group

The s command has three parts:

  1. The command s itself
  2. What to match, including definiton of groups: \(.*\)/.*
  3. What to output i place of what was matched: \1

So this takes everything up the last / as the first group and prints it by referencing it with \1.

Upvotes: 1

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