How to use \l and \u in regex mathing patterns?

[pat_mc]

Hi there, esteemed Monks!

I am having trouble with the upper and lower case backslash escapes in regular expressions and would very much appreciate your help with these.

So far, I thought \l matches lowercase characters, similar to [a-z] would for English text. This is obviously wrong:

    $ perl -e '( $a = "hello" ) =~ s/^\l//; print $a'
    hello

I then thought it acts like an operator to modify substitution strings:

    $ perl -e '( $a = "Hello" ) =~ s/(.+)/\l$1/; print $a'
    hello

This seems to work. An analogous result is obtained for \u

My question is: Can I used those escape characters in a matching pattern as well - and if so, what is their meaning in that context?

Really appreciate your wisdom.

Cheers -

Pat