regex=''

# This expression generates links to commits referenced by their SHA1.
regex=$regex'
s|\b([0-9a-fA-F]{7,64})\b|<a href="./?id=\1">\1</a>|g'

# This expression generates links to a fictional bugtracker.
regex=$regex'
s|#([0-9]+)\b|<a href="http://bugs.example.com/?bug=\1">#\1</a>|g'

sed -re "$regex"