Question

I’m trying to replace values which have a trailing minus sign and replace them with a leading minus sign. I’ve got this regex which doesn’t quite work in SED though I don’t understand why.

echo '8.8-' | sed 's/(\d+(?:\.\d+)?)-/-\\1/'

SED returns 8.8-

I tried the expression in Notepad++ RegEx Help plugin which matches the 8.8- and identifies the sub-match \1 as 8.8

Answer 1

The \d is a Perlism. Sed refers to the older, pre-Perl, POSIX standard. What you probably want is

echo '8.8-' | sed -r 's/([[:digit:]]+(\.[[:digit:]]+)?)-/-\1/'

The [[:digit:]] is the POSIX way to write Perl’s \d.

Of course, you can just write [0-9], instead, as others have noted.

Incidentally, you need the -r to make your parentheses work as you like.