**tl;dr** see [fd0][1]'s solution

I did look in the man page before asking, but failed to notice the relevant info, since I figured it doesn't differ significantly from that of Linuxes. But,

 $ man sed
 ...
 -E Interpret regular expressions as extended (modern) regular
 expressions rather than basic regular expressions (BRE's). The
 re_format(7) manual page fully describes both formats.
 $ man re_format
 ...
 Obsolete (“basic”) regular expressions differ in several respects. ‘|’
 is an ordinary character and there is no equivalent for its functional‐
 ity. ‘+’ and ‘?’ are ordinary characters, and their functionality can be
 expressed using bounds (‘{1,}’ or ‘{0,1}’ respectively). Also note that
 ‘x+’ in modern REs is equivalent to ‘xx*’. The delimiters for bounds are
 ‘\{’ and ‘\}’, with ‘{’ and ‘}’ by themselves ordinary characters.

And I did try to use `{1,}`, I just forgot to escape curly brackets. So surely [fd0][1]'s solution is generally the best way possible. But others would be:

 $ echo ' found' | sed -n '/[[:blank:]]\{1,\}/p'
 found
 $ echo ' found' | sed -n '/[[:blank:]][[:blank:]]*/p'
 found

However, not sure what they meant by

> Also note that ‘x+’ in modern REs is equivalent to ‘xx*’

Meaning, what it was like in nonmodern ones.

Also, `grep`'s manpage didn't mention `re_format` entry, which apparently means it has its own implementation of regular expressions. For that matter, it was GNU `grep`, as opposed to `sed`.

[1]: http://unix.stackexchange.com/a/233115/29867