Order of terms in square braces

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

Brethern,

I'm running Perl 5.8.6. I'm trying to pattern match some comma-delimited phrases in text. The phrases can have multiple words, plus the following punct:
( ) < > -

I'm using square braces in a regex, and seeing the following behavior that I don't understand (running in the interpreter within the debugger):

  DB<116> x 'cold, too cold' =~ /( [\(\)-<>\w]+ )/x
0  'cold,'
  DB<117> x 'cold, too cold' =~ /( [-<>\w\(\)]+ )/x
0  'cold'
[download]

Why am I matching the trailing comma in the first case but not the second? Is the matching within square braces supposed to be order-dependent? None of the refs I've looked at mention it.

throop

Comment on Order of terms in square braces Download Code

Replies are listed 'Best First'.
Re: Order of terms in square braces by imp (Priest) on Feb 03, 2007 at 00:44 UTC
Because dashes in character classes are used to express ranges, like `[a-z]`, except when they are the first item. Because of this you should always either put them as the first item, or escape them with \	[reply] [d/l]
Re^2: Order of terms in square braces by TOD (Friar) on Feb 03, 2007 at 05:10 UTC
first or last element in the class.	[reply]
Re^3: Order of terms in square braces by ikegami (Patriarch) on Feb 03, 2007 at 07:12 UTC
or following a POSIX class or preceeding a POSIX class or following `^` if the `^` is the first character etc Basically, anywhere it doesn't make sense to treat it as a range seperator. Escaping `-` (like the OP needlessly did for `(` and `)`) will force it to match literally.	[reply] [d/l] [select]
Re^2: Order of terms in square braces by throop (Chaplain) on Feb 05, 2007 at 19:37 UTC
d'Oh. I should have known that. Thanks. throop `The people are bright and certain, Where I am dim and confused; The people are clever and wise, Where I am dull and ignorant,` [download]	[reply] [d/l]
Re: Order of terms in square braces by BrowserUk (Patriarch) on Feb 03, 2007 at 00:51 UTC
In the first case, this part of your character class `\)-<` is a character range. Ie. It is equivalent to all the characters between ')' (ascii 41) and '<' (ascii 60), which includes ascii 44, ','. In the second case, by placing the '-' as the first character inside the brackets, you've disabled the range function and it is treated as a normal character, Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. "Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."	[reply] [d/l]


Keep It Simple, Stupid
	PerlMonks