Using bash (parameter expansion) to sanitize input file - regex

Question

I have a bash script that has a function like so:
sanitize(){
rb_reg="^([0-9]{1,3})\.([0-9]{1,3})\.([0-9]{1,3})$"
if grep -Ex "${rb_reg}" "${1}/.ruby-version" > /dev/null 2>&1; then
sanitize_tmp="$(<"${1}"/.ruby-version)" &&
ruby_version="${sanitize_tmp//[^0-9\.]/}" &&
echo "Setting Ruby Version: ${ruby_version}"
else
echo "There was an error trying to sanitize a .ruby-version file"
echo "The file was: ${1}/.ruby-version"
exit 7
fi
}
I'm using it to check a .ruby-version file and then set the version in there as a variable.
Mostly these files will contain something sensible like: 2.0.0 which works OK. I want to be defensive and not trust the input file, so check/sanitize it as much as possible.
Two questions:
If for some reason there were multiple version numbers in the file on multiple lines, say:
'2.0.0
1.0.0'
That's going to smash them together currently removing white space and end up with a variable like: '2.0.01.0.0'
What's a good way to only pick up the first version number that matches the regex?
Is there a better way to do this, maybe entirely in bash without grep? Appreciate any examples people have of checking for a version like this but not trusting the input file.

Answer 1

I'm still playing around with this a little, but here is what I ended up doing.
I'm passing in the file name as an argument to the function elsewhere in the script. Really liked the concept of BASH_REMATCH, so tried to avoid using grep, sed, awk etc and do it this way.
You can view the latest version of the code here: https://github.com/octopusnz/scripts
sanitize(){
if [[ "${#}" -ne 1 ]]; then
echo "[ERROR 7]: We expected 1 argument to the sanitize() function."
echo "But we got ${#} instead."
exit 7
fi
rbv_reg="^([0-9]{1,2})\.([0-9]{1,2})\.([0-9]{1,2})(-([a-z]{1,10}))?$"
reg_matches=0
while read -r rbv_line || [[ -n "$rbv_line" ]]; do
if [[ "${rbv_line}" =~ ${rbv_reg} ]]; then
ruby_version="${BASH_REMATCH[0]//[^0-9a-z\.\-]/}" &&
((reg_matches="${reg_matches}"+1)) &&
echo "" &&
echo "Setting Ruby version: ${ruby_version}" &&
break
fi
done < "${1}"
if [[ "${reg_matches}" -lt 1 ]]; then
if [[ -v ruby_version ]]; then
echo "We couldn't parse ${1} and set a valid Ruby version."
echo "Using default: ${ruby_version}"
else
echo "We couldn't parse ${1} and set a default Ruby version."
echo "[ERROR 4]: No valid .ruby-version file found."
exit 4
fi
fi
}