regex - Beginning and end of words in sed and grep

Question

I don't understand the difference between and < in GNU sed and GNU grep. It seems to me can always replace < and \> without changing the set of matching strings.

More specifically, I am trying to find examples in which something and \< something do not match exactly the same strings.

Same question for something and something\>.

Thank you

Answer 1

I suspect that it very rarely makes a difference whether you use (the more common) or (the more specific) < and >, but I can think of an example where it would. This is quite contrived, and I suspect that in most real-world regex use it wouldn't make a difference, but this should demonstrate that it at least could make a difference in some cases.

If I have the following text:

this is his pig

and I want to know if /is/ matches, it wouldn't matter if I instead used /<is>/ or I instead used />is</

But what if my text was instead

is this his pig

There's no longer a word-final boundary before the 'is', only a word-initial boundary. Using /is/ matches, and of course /<is>/ does too, but />is</ does not.

In real life, though, I think it is not common that you really need to be able to make this distinction, which is why (at least outside of sed) is the normal word boundary marker for regular expressions.