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

pat_mc has asked for the wisdom of the Perl Monks concerning the following question:

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
[download]

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

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

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

Comment on How to use \l and \u in regex mathing patterns? Select or Download Code

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Re: How to use \l and \u in regex mathing patterns? by AnomalousMonk (Archbishop) on May 09, 2012 at 16:36 UTC
The `\l` and `\u` (and also `\L \U`) operators are interpolation control operators. For upper- and lower-case regex character sets, see `[[:upper:]]` and `[[:lower:]]` among others, e.g., the Unicode classes. See 'Escape Sequences' section in perlre, also perlrecharclass.	[reply] [d/l] [select]
Re: How to use \l and \u in regex mathing patterns? by MidLifeXis (Monsignor) on May 09, 2012 at 16:31 UTC
From perlre, `\l` lowercases the next character, , and `\L` lowercases until the next `\E`. It does not match anything. --MidLifeXis	[reply] [d/l] [select]
Re: How to use \l and \u in regex mathing patterns? by Anonymous Monk on May 09, 2012 at 16:41 UTC
See perlrecharclass, perluniprops. `$ perl -e'print "hello" =~ /\p{Lowercase}+/' 1 $ perl -e'print "HELLO" =~ /\p{Uppercase}+/' 1 $ perl -e'print "Hello" =~ /\p{Uppercase}\p{Lowercase}+/' 1` [download]	[reply] [d/l]
Re: How to use \l and \u in regex mathing patterns? by brx (Pilgrim) on May 09, 2012 at 16:45 UTC
`$ perl -e '$b="H";( $a = "hello" ) =~ s/^\l$b/Y/; print $a' Yello` [download] see perlre: `\l lowercase next char` So, after interpolation, "\l$b" gives "\lH" ie "h" and you obtain `s/h/Y/`	[reply] [d/l] [select]
Re^2: How to use \l and \u in regex mathing patterns? by Anonymous Monk on May 09, 2012 at 16:54 UTC
What an ugly hack. If someone asks "How do I X with Y?", and you recognise that Y is a particularly bad way to accomplish X, then you are supposed to answer with the better way Z. If taking the question at face value, at least mention the caveats of Y.	[reply]
Re^3: How to use \l and \u in regex mathing patterns? by AnomalousMonk (Archbishop) on May 09, 2012 at 17:06 UTC
I don't think it was a hack, ugly or otherwise. I think it was intended, and succeeds, as an example of the actual way interpolation control operators can work in a regex, which was the context of the original – albeit misguided, since pat_mc thought of them as character classes – question. (Although I must admit that, had I given the example myself, I might have mentioned that those operators are rarely used in that particular way.)	[reply]
Re^3: How to use \l and \u in regex mathing patterns? by Anonymous Monk on May 09, 2012 at 17:18 UTC
If someone asks ... Somebody already gave that answer	[reply]
Re^3: How to use \l and \u in regex mathing patterns? by JavaFan (Canon) on May 09, 2012 at 18:26 UTC
... then you are supposed to ... No, you are not.	[reply]
Re: How to use \l and \u in regex mathing patterns? by sundialsvc4 (Abbot) on May 10, 2012 at 12:10 UTC
I certainly don’t find the oblique comment, “Think `vi`” in `perldoc perlre` to be particularly informative ... but then again, I don’t regularly use `vi`. I therefore cordially suggest that I have never used them or paid any attention to them, and I do feel that I am none the worse for wear. I would use character-classes and other such strategies to accomplish the intended result, and studiously avoid these. But, that’s just me, perhaps. I tend to find a way that works and that makes “obvious” sense to me, and that’s the end of it.

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