Re: Need help with Perl regex
From: Eric Bohlman (ebohlman_at_omsdev.com)
Date: 01/07/05
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Date: 7 Jan 2005 02:45:15 GMT
surfking <bkimelman@JUNK.sympatico.ca> wrote in
news:MPG.1c4798934e49976498986f@news1.on.sympatico.ca:
>
>
> I found this line of code
> which was parsing the /etc/termcap file located on a UNIX system.
>
> if (/(^|\|)${term}[:\|]/) {
>
> I used the code from which this line was extracted and it successfully
> parsed/extracted the termcap entry for my particulat type of terminal.
>
> I realize that the "^" character is used to anchor the pattern match
> to the start of a buffer and that enclosing part of a pattern match
> within a set of parenthesis enables you to retrieve the value of the
> matched segment and that "|" is used as an "logical or" operator, but
> given the format of entries in the /etc/termcap file, I don't see how
> this pattern is successfull. Can anyone out there give me some ideas
> on this ?
Actually, in this case the parentheses are almost certainly being used
simply to set precedence.
Let's spread that regex out a bit, which we can actually do in Perl code
thanks to the "x" modifier:
/ #start regex
( #begin group that's treated as a unit
^ #start of the string
| #logical or
\| #a literal pipe character
) #end group
#so in order to match, it has to either be at the beginning of the line
#or preceded by a pipe symbol
${term} #treat whatever is in the variable $term as part of the regex
[ #begin a character class
: #a literal colon
\| #a literal pipe character
] #end character class
#the character class matches any character that's either a colon or a
#pipe /x #end regex; the "x" lets us put in spaces and comments
So we know that whatever matches has to come either at the beginning of
the line or after a pipe symbol, and it has to end with a colon or a
pipe. The question is, what's in between? We can't know the answer
until we know what's in $term. I can guess (only guess) that it's
simply the name of your terminal and doesn't contain any regex special
characters. If that's the case, then the expression will match any line
in which the name of your terminal appears either at the beginning or
after a pipe, and is immediately followed by either a colon or a pipe.
But again, that's just a guess; if $term contrains any regex special
characters, they'll be treated the same as if they had been written out
in the regex.
The perlretut, perlrequick, perlre, and perlreref documents that come
with every Perl distribution are the definitive reference for Perl
regexes. Start with:
perldoc perlretut
and work your way through them.
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