as promised here is my solution. Past this in your commit.msg hook and adapt to your needs.
#!/bin/bash
# give me the branch I am currently working on
mybranch=`git rev-parse --abbrev-ref HEAD`
# Depending on the branch we do different validation
if [ $mybranch = master ]; then
regex="^[^Merge|^#].{71,}"
message="ERROR: Commit message is too short."
else
regex="^(PMS|GKM)-[0-9]{3,4}\s\:\s.{10,}"
message="ERROR: Commit message is missing the Ticketnumber in the format GKM-nnnn: or PMS-nnnn :."
fi
test "" != "$(grep -E $regex "$1")" || {
cat $1 >&2
echo >&2 $message
exit 1
}
Depending on the branch I choose different regular expressions and matching error messages.
This ensures for any branch not being master, that the message starts with a ticket number. For the master branch we don't need a ticket number, but a longer commit message.