I think the Linux expr command is woefully underused for matching and extraction of substrings using basic regular expressions, so herewith a quick primer.

Consider a fully-qualified package name with version number and, say, patch level number:

$ P="fubar-1.2.3-42"

Say one wanted to match or extract different components of that string – this variation of the expr command will, first, simply count the number of matching characters based on an anchored match; that is, anchored to the beginning of the string.

Let's count the length of the initial substring that matches anything but a hyphen:

$ expr $P : '[^-]*'
5
$

Questionably useful, I guess, so let's extend this by counting the number of characters in the combined package number and version (and intervening hyphen):

$ expr $P : '[^-]*-[^-]*'
11
$

Finally, we could, of course, use that trick to match the entire string based on the pattern we know it will have (allowing a simple “.*” to suck up the rest of the string):

$ expr $P : '[^-]*-[^-]*-.*'
14
$

“So what?”, you think. Ah, but you can also tag the field you care about to have that field value printed, not simply its length.

Extract the package name:

$ expr $P : '\([^-]*\)'
fubar
$

Extract the version number:

$ expr $P : '[^-]*-\([^-]*\)'
1.2.3
$

Finally, extract the patch level:

$ expr $P : '[^-]*-[^-]*-\(.*\)'
42
$

Piece of cake.