block_branch_names.sh

#!/usr/bin/env bash

#
# Pre-receive hook that will block any new commits that their names contain
# other than lower-case alphabet characters (a-z).
#
# More details on pre-receive hooks and how to apply them can be found on
# https://help.github.com/enterprise/admin/guides/developer-workflow/managing-pre-receive-hooks-on-the-github-enterprise-appliance/
#

zero_commit="0000000000000000000000000000000000000000"

# Ensure that [a-z] means only lower case ASCII characters => set LC_COLLATE to 'C'
# See http://unix.stackexchange.com/questions/227070/why-does-a-z-match-lowercase-letters-in-bash
# See https://www.gnu.org/software/bash/manual/bashref.html#Pattern-Matching
LC_COLLATE='C'

while read oldrev newrev refname; do
  # Only check new branches ($oldrev is zero commit), don't block tags
  if [[ $oldrev == $zero_commit && $refname =~ ^refs/heads/ ]]; then
    # Check if the branch name is lower case characters (ASCII only), '-', '_', "/" or numbers
    if [[ ! $refname =~ ^refs/heads/[-a-z0-9_/]+$ ]]; then
      echo "Blocking creation of new branch $refname because it must only contain lower-case alpha-numeric characters, '-', '_' or '/'."
      exit 1
    fi
  fi
done
exit 0