Bash style environment variable expander in Python

At work we have a certain config file that has bash/tcsh style embedded environment variable references of two styles:
${VARNAME}
and
$VARNAME

I thought I would try my hand at a concise python expander for the syntax and came up with:

bash$  python -c '
import sys,os,re
s=sys.stdin.read()
s1 = re.subn(r"\$([a-z_A-Z][A-Za-z_0-9\-]*)", r"%(\1)s", s)[0]
s2 = re.subn(r"\${([^}]+)}", r"%(\1)s", s1)[0]
print s2 % os.environ ' < in_file > expanded_file