Special Variable Pattern Match whilst reading input from a file

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

Please forgive me - I need help! I'm a little rusty and I'm wondering if there's a better way of achieving the result I'm after.

I need to extract a string from a line within file. I will then use this string as a variable within my procedure.

The line always starts with the string "frz = ", and the value I need to extract comes after this.

Here's what I've got so far:

open (CONFIG_FILE, "$config_file") || die

while (<CONFIG_FILE>) {
  if (($foo) = /^frz\s+=\s+(\w+)\;/) {
    $bar = $foo;
  }
}
print "$bar\n";
close (CONFIG_FILE) || die
[download]

So, I end up with $bar as the variable I can use.

My question is "Do I need $foo as an intermediate variable?" Is there a more 'elegant way of extracting the information I want?

Comment on Special Variable Pattern Match whilst reading input from a file Download Code

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Re: Special Variable Pattern Match whilst reading input from a file by zby (Vicar) on Mar 29, 2003 at 19:49 UTC
`while (<CONFIG_FILE>) { if(/^frz\s+=\s+(\w+)\;/) $bar = $1; } } print "$bar\n";` [download] You can use the `$n` variables.	[reply] [d/l] [select]
Re: Special Variable Pattern Match whilst reading input from a file by Enlil (Parson) on Mar 29, 2003 at 20:04 UTC
A couple of things. 1. `/^frz\s+=\s+(\w+)\;/` will not match a line starting with `"frz="` as there is a mandatory whitespace after the z in your regular expression (you were probably looking for `\s` so it will match both lines starting with `"frz="` or `"frz ="` (you might want to change the other `\s+` to `\s` depending on what the line might look like after the "=". 2. One way to skip the intermediate variable would be: `open (CONFIG_FILE,"$config_file") or die "Can't open $config_file"; my $bar; while(<CONFIG_FILE>){ next unless /^frz\s=\s(\w+)\;/; $bar = $1; } print $bar,$/; close(CONFIG_FILE) or die;` [download] -enlil	[reply] [d/l] [select]
Re: Special Variable Pattern Match whilst reading input from a file by nothingmuch (Priest) on Mar 29, 2003 at 19:53 UTC
`open (CONFIG_FILE, "$config_file") \|\| die LINE: while (<CONFIG_FILE>) { if ( /^frz\s+=\s+(\w+)\;/ ) { $bar = $1; } } print "$bar\n"; close (CONFIG_FILE) \|\| die;` [download] I'd like to note that currently bar will simply be set to the last matching line's value. Is that what you want? because you can also add a `last LINE;` expression, under `$bar = $1`, to tell perl you're done reading. You needn't go through the whole file, only up to the point where the line is. -nuffin zz zZ Z Z #!perl	[reply] [d/l] [select]
Re: Special Variable Pattern Match whilst reading input from a file by dakkar (Hermit) on Mar 29, 2003 at 19:53 UTC
Parenthesised matches always end up in the match variables `$1, $2, ...`, so you could write: `while (<CONFIG_FILE>) { if (/^frz\s+=\s+(\w+);/) {$bar=$1} }` [download] If you have more than one pair of parenthesis, `$1` will refer to the match of the first pair, `$2` to the second and so on. The pairs are ordered from left to right, counting their opening parenthesis, so: `"abc123def"=~/([a-z]+(\d+)(.*))/; print "first: $1\nsecond: $2\nthird: $3\n";` [download] would print: first: abc123def second: 123 third: def -- dakkar - Mobilis in mobile	[reply] [d/l] [select]
Re: Special Variable Pattern Match whilst reading input from a file by DarknessX (Scribe) on Mar 29, 2003 at 19:53 UTC
try this: `open FILE, "$file" or die "$!"; while (<FILE>){ m/^frz\s=\s(\w+)\;/; print $1; } close FILE;` [download] See, the (\w+) will take whatever is matched by \w+ and store it in `$1`, perl's magic match variable. that way you eliminate the need for `$foo and $bar` altogether. `-----BEGIN GEEK CODE BLOCK----- Version: 3.1 GPA/P/S d+ s++:+ a--- C++ UL++ P++>++++ L++ E W++ N o? K? w-- !O M- V? + PS++ PE- Y PGP- t 5- X+ R* tv-- b++ !DI D---(+) G e- h-- r++ y+++++> ++ ------END GEEK CODE BLOCK-----` [download]	[reply] [d/l] [select]
Re: Special Variable Pattern Match whilst reading input from a file by Aragorn (Curate) on Mar 29, 2003 at 20:23 UTC
And a slightly different solution: `while (<CONFIG>) { $bar = $1 if /^frz\s=\s(\w+);/; }` [download] Arjen Update:Oops, forgot the (apparently) mandatory semicolon.	[reply] [d/l]


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