weird regex question

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

so i asked a while ago for an regex alternative to substr.. here is 1 of the replies:

$string = "hello world";
print $string =~ /^.{1}(.{5})/;
[download]

also i know that !$! = 1, so i tried this:

$string = "hello world";
print $string =~ /^.{!$!}(.{5})/;
[download]

that would produce an empty line, why? I have tried many different ways but seems im restricted to using digits. just out of curiosity, why cant i do this?

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Re: weird regex question by kennethk (Abbot) on Apr 25, 2012 at 16:17 UTC
You can get an explanation of any regular expression using YAPE::Regex::Explain. In this case, it outputs: The regular expression: (?-imsx:^.{!}(.{5})) matches as follows: NODE EXPLANATION ---------------------------------------------------------------------- (?-imsx: group, but do not capture (case-sensitive) (with ^ and $ matching normally) (with . not matching \n) (matching whitespace and # normally): ---------------------------------------------------------------------- ^ the beginning of the string ---------------------------------------------------------------------- . any character except \n ---------------------------------------------------------------------- {!} '{!}' ---------------------------------------------------------------------- ( group and capture to \1: ---------------------------------------------------------------------- .{5} any character except \n (5 times) ---------------------------------------------------------------------- ) end of \1 ---------------------------------------------------------------------- ) end of grouping ---------------------------------------------------------------------- [download] The issue is that the `$!` variable is interpolated to an empty string but `!` operator doesn't get interpolated as such in the regular expression. Essentially, your modification is actually looking for the string literal `{!}`. #11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.	[reply] [d/l] [select]
Re: weird regex question by JavaFan (Canon) on Apr 25, 2012 at 16:25 UTC
If you use `!$!`, only `$!` get interpolated, not the entire expression. In your case, `$!`, in string context, will be the empty string. So, you end up with `/^.{!}(.{5})/` [download] which is not going to match. If you want to interpolate the expression, use: `/^.{@{[!$!]}}(.{5})/` [download]	[reply] [d/l] [select]
Re^2: weird regex question by Anonymous Monk on Apr 25, 2012 at 17:07 UTC
hmm kinda makes sense, so how would i do the 5?	[reply]
Re^3: weird regex question by JavaFan (Canon) on Apr 25, 2012 at 17:12 UTC
What do you mean, how would I do the 5?	[reply]
Re^4: weird regex question by Anonymous Monk on Apr 25, 2012 at 17:13 UTC
Re^3: weird regex question by trizen (Hermit) on Apr 25, 2012 at 20:47 UTC
1 and 5 `my $string = "hello world"; print $string =~ /^.{@{[!$[]}}(.{${\($]<<$]>>$])}})/; # 'ello '` [download] or `print $]>>$?;`	[reply] [d/l] [select]