regex

sed.txt

# https://stackoverflow.com/questions/15965073/return-code-of-sed-for-no-match
# https://unix.stackexchange.com/questions/145402/regex-alternation-or-operator-foobar-in-gnu-or-bsd-sed

# POSIX ERE + GNU
echo "   13.0-10-g692d0a9 d_2017_09_14 " | sed -E '/^\s*([0-9.-]+g(\S+)|([0-9.]+)\+?[0-9]*).+/,${s//\2\3/;b};$Q1')

# pure POSIX ERE -- does not work on OSX
echo "   13.0-10-g692d0a9 d_2017_09_14 " | sed -E '/^[[:space:]]*[[:digit:].-]{1,}g([[:xdigit:]]{1,})[[:space:]]{1,}.*/!{/^[[:space:]]*([[:digit:]\.]{1,})(+[[:digit:]]{1,}){0,1}[[:space:]]{1,}.*/!b};s//\1/'

# python 2.7 / 3 compat -- can not be done on oneline?
echo "   13.0-10-g692d0a9 d_2017_09_14 " | python -c "
import sys,re;
for line in sys.stdin:
  foo = re.sub(r'^\s*(?:[\d.-]+g(\S+)|([\d.]+)\+?[\d]*)\s+.*', lambda m: m.group(1) or m.group(2), line)
  if foo is line:
    sys.exit(1)
  print(foo)
"