Convert bash script using regexes to sh - linux

Question

I'm really struggling with writing a Bourne shell script. Basically, I have three input formats for a variable called "ref" that I'm trying to detect:
ref="refs/head/.*" (i.e. begins with "refs/head/" I'm interested in the bit at the end, after the slash)
ref="refs/tags/.*" (i.e. begins with "refs/tags/" I'm interested in the bit at the end, after the slash)
everything else (i.e. ignore everything that doesn't begin with either "refs/head/" or "refs/tags/")
For example,
If ref="refs/head/master", set TAG="master"
If ref="refs/tags/0.2.4", set TAG="0.2.4"
For everything else, set TAG=""
Now I wrote something in bash shell, but I'm really struggling to convert it to Bourne (#!/bin/sh):
#!/bin/bash
#
#This works!
#
TAG=""
re='^refs/head/.*' #regex: begins with refs/head/, ends with anything
re2='^refs/tags/.*' #regex: begins with refs/tags/, ends with anything
if [[ $ref =~ $re ]]; then
#do nothing - OK
true #NOP
else
#check if it's a tag update
if [[ $ref =~ $re2 ]]; then
TAG=${$ref##*/} #looks worse that it is: http://stackoverflow.com/questions/3162385/how-to-split-a-string-in-shell-and-get-the-last-field
fi
exit 0
fi
echo $TAG
It took me ages to a) write this program and b) find out why my program was going nuts - turned out I need #!/bin/sh and not #!/bin/bash
How can I convert this to sh? Maybe someone else has a more elegant solution to my regex gymnastics?
Update:
Thanks for the answers sofar (especially #gboffi). I think I'm almost there.
All I need now is to know if $TAG comes from "refs/head/", "refs/tags/" or neither. I tried to modify some of the answers, but really struggling with sh. I'll need to go away and learn more about sh from first principles instead of trying to hack it.
Update 2:
So after a night's sleep, I figured it out in about 20 minutes. Here is my solution:
#!/bin/sh
ref="refs/asdf/master"
TAG=""
TAG="${ref#refs/heads/}"
if [ "$ref" != "${ref#refs/heads/}" ]; then
echo "heads"
echo $TAG
else
TAG="${ref#refs/tags/}"
if [ "$ref" != "${ref#refs/tags/}" ]; then
echo "heads"
echo $TAG
else
TAG=""
fi
fi
echo "--->$TAG"
I'm sure there's a much more elegant solution; but I just don't have the time!

Answer 1

Here it is a function, defined in dash (the linux version of sh)
% dash
$ tag() {
> TAG=""
> [ "$1" != "${1#refs/head/}" ] && TAG="${1#refs/head/}"
> [ "$1" != "${1#refs/tags/}" ] && TAG="${1#refs/tags/}"
> echo "$TAG"
>}
$ tag poldo
$ tag refs/head/poldo
poldo
$ tag refs/tags/pippo
pippo
$ tag to093u0refs/head/poldo
$exit
%

Answer 2

The most idiomatic way to handle this is with a case statement and pattern matching:
#!/bin/sh
case $ref in
refs/head/*) true ;; # original behavior: no output, success
refs/tags/*) printf '%s\n' "${ref##*/}" ;; # original behavior: emit output
*) false # exit with an error # original code didn't specify behavior
esac
The reason I'm exiting with an error for the case where neither refs/head/* or refs/tags/* matches is that if you wanted to exit with success in that case, you could omit the refs/head/* test entirely.

Answer 3

You can use rev-parse for this, here are some examples from one of my
repositories. Notice the last one produces no output, as desired.
$ git rev-parse --abbrev-ref refs/heads/master
master
$ git rev-parse --abbrev-ref refs/tags/5
5
$ git rev-parse --abbrev-ref #^

Answer 4

Having read your description of the desired function it seems that it boils down to this:
check the input argument. If it starts with refs/head or refs/master, proceed, else stop.
take the terminal piece of information and set the variable TAG to it.
thus, assuming your input is in the variable REF,
TAG=""
echo $REF | egrep -q 'refs/head|refs/master'
if [ $? -eq 0 ]
then
TAG=`echo $REF | sed "s/.*\///"`
fi
ought to do it.
(Side note for shell heads: is basename a better solution than sed in this particular case?)