Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!crdgw1!uunet!rbj
From: rbj@uunet.UU.NET (Root Boy Jim)
Newsgroups: comp.lang.perl
Subject: Re: Puzzle
Message-ID: <127421@uunet.UU.NET>
Date: 3 Apr 91 19:52:23 GMT
References: <1991Apr1.224210.28384@uvaarpa.Virginia.EDU> <1991Apr3.021455.27260@eng.umd.edu>
Organization: UUNET Communications Services, Falls Church, VA
Lines: 55

ziegast@eng.umd.edu (Eric W. Ziegast) writes:
>worley@compass.com writes:
>>The following program:
>>
>>    $f = '/usr';
>>    $a = ($f =~ m#/usr($|/)#);
>>    $b = ($f =~ m#${f}($|/)#);
>>    print "\$a = '$a'\n";
>>    print "\$b = '$b'\n";
>>
>>produces the output:
>>
>>    $a = '1'
>>    $b = ''
>>
>>Why are the two apparently similar pattern matches doing different
>>things?  Is ${f} absorbing the following parenthesized pattern as a
>>subscript?  If so, how is it succeeding, since "$|/" is not a valid
>>expression?  Am I going crazy?
>
>Variables, as far as I know, are *not* evaluated in expressions.
>Note what $ is really used for in expressions, end of expression.

Of course they are! That's what variables are for!
I assume you meant "vars are not eval'ed in patterns (m,s,tr,split)".

Well, the answer is different for each one. Split and m// treat
patterns as if they were double quoted, that is, interpolation
occurs. tr/// treats them as single quoted, making them literal.
Last but most, s/// will do either, depending on whether or not
the slash is a single quote or anything else, except when
backquotes are used, in which case command substitution occurs,
except on Tuesday or at night if perl is installed in /phis/bin.

For your added pleasure, a variety of flags are available.
Perhaps someday Larry will unify these a bit more.

>The ${f} is what causes a problem for $b.  The $| is not evaluated
>either, but it is not clear in what context you are using it.

In regular expressions, $| and $) are not treated as variable
references, but as separate characters; the $ means end of line
and the (|) are used for alternation of patterns.

>In order to evaluate variables inside your regex, you have to use
>'eval' to evaluate the variables first before the expression.

Please check your facts before you post.

BTW, while talking to Merlyn on the phone I heard him mumble
"perl -de 0" in order to get an interactive debugger. Since I
haven't seen this tip in the book, I thought I'd pass it on.
-- 
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