Remove substring matching pattern both in the beginning and the end of the variable

Question

As the title says, I'm looking for a way to remove a defined pattern both at the beginning of a variable and at the end. I know I have to use # and % but I don't know the correct syntax.

In this case, I want to remove http:// at the beginning, and /score/ at the end of the variable $line which is read from file.txt.

Solution 1

Well, you can't nest ${var%}/${var#} operations, so you'll have to use temporary variable.

Like here:

var="http://whatever/score/"
temp_var="${var#http://}"
echo "${temp_var%/score/}"

Alternatively, you can use regular expressions with (for example) sed:

some_variable="$( echo "$var" | sed -e 's#^http://##; s#/score/$##' )"

Solution 2

$ var='https://www.google.com/keep/score'
$ var=${var#*//} #removes stuff upto // from begining
$ var=${var%/*} #removes stuff from / all the way to end
$ echo $var
www.google.com/keep

Solution 3

You have to do it in 2 steps :

$ string="fooSTUFFfoo"
$ string="${string%foo}"
$ string="${string#foo}"
$ echo "$string"
STUFF

Solution 4

There IS a way to do it one step using only built-in bash functionality (no running external programs such as sed) -- with BASH_REMATCH:

url=http://whatever/score/
re='https?://(.*)/score/'
[[ $url =~ $re ]] && printf '%s\n' "${BASH_REMATCH[1]}"

This matches against the regular expression on the right-hand side of the =~ test, and puts the groups into the BASH_REMATCH array.

---

That said, it's more conventional to use two PE expressions and a temporary variable:

shopt -s extglob
url=http://whatever/score/
val=${url#http?(s)://}; val=${val%/score/}
printf '%s\n' "$val"

...in the above example, the extglob option is used to allow the shell to recognized "extglobs" -- bash's extensions to glob syntax (making glob-style patterns similar in power to regular expressions), among which ?(foo) means that foo is optional.

---

By the way, I'm using printf rather than echo in these examples because many of echo's behaviors are implementation-defined -- for instance, consider the case where the variable's contents are -e or -n.

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Comments

[Admin]

What kind of pattern? Can you be a bit more specific?

[bobylapointe]

yes, in this case: http:// at the beginning, and /score/ at the end. The $line is read from a file.txt

[doubleDown]

What you are looking for is parameter expansion, specifically to your case the ${parameter#word} and ${parameter%word} near the end of this section of Bash manual

[bobylapointe]

I'm way more familiar with regular expressions. This is interesting. I know sed a bit. The question is, how to use the output produced by sed to use it in another command. Let's say to start a script. ./script.sh $variablemodifiedbysed

[Admin]

Updated the answer to show that the output of sed can be stored in variable, for reuse.

[bobylapointe]

This finally works like a charm. I've been spending two hours figuring this out. I'm sure the others work as well, but again, I like using regular expressions. Thanks man !

[jaypal singh]

@bobylapointe Don't forget to mark this answer by clicking the check mark. :)

[Charles Duffy]

Why use sed when bash has regular expression matching built-in?

[Nazar]

In my case, the string may or may not start with 'boo' or 'foo' and if it starts with any of them how could I implement an OR into the parsing to remove "boo" OR "foo" from beginning of a string?

[axolotl]

what are these called and how do I learn more about such hacks in bash string manip?