Question

echo "a b _c d _e f" | sed 's/[ ]*_[a-z]\+//g'

The result will be a b d f.

Now, how can I turn it around, and only print _c _e, while assuming nothing about the rest of the line?

Answer 1

If the question is “How can I print only substrings that match specific a regular expression using sed?” then it will be really hard to achieve (and not an obvious solution).

grep could be more helpful in that case. The -o option prints each matching part on a separate line, -P enables PCRE regex syntax:

$> echo "a b _c d _e f" | grep -o -P "(\ *_[a-z]+)"
 _c
 _e

And finally

$> echo `echo "a b _c d _e f" | grep -o -P "(\ *_[a-z]+)"`
_c _e