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Re: replace/substituion 4th field

by 7stud (Deacon)
on Feb 20, 2013 at 18:10 UTC ( #1019817=note: print w/ replies, xml ) Need Help??


in reply to replace/substituion 4th field

$line =~ s/(.*)\,(.*)\,(.*)\,(.*)\,(.*)\,(.*)/

Why are you escaping all those commas? A comma has no special regex meaning. In a regex, a comma means one thing: a literal comma.

The pattern .* says to match ANY character, 0 or more times, GREEDILY. That means that the regex engine will match as many characters as possible. When the regex engine sees a pattern like:

(.*),

...the first thing the regex engine does is find a match for the part in parentheses. Your whole line matches that part, so the regex engine reads in your whole line as the match for (.*). Then the regex engine moves on to the comma. Because there are no characters remaining in the line to match against, the regex engine backs up one character, surrendering a character from what matched .*. Then the regex engine checks if the comma matches that character. No match. So the regex engine backs up another character, surrendering yet another character from what matched .*, and the regex engine checks again if that character matches the comma. So on and so on until the regex finds a match for the comma. That's inefficient.

A more efficient regex would be this:

([^,]+),([^,]+)

However, as tmharish already mentioned you already split() your data, so there is no reason to use a regex at all. Just change whatever pieces you want in @fields like this:

$fileds[3] = "hurray";

You can limit your split to 5 instead of on every comma, which will speed things up a little.

It is possible to do a conditional s///, like this:

$line =~ s/ ( (?: [^,]+ , ){3} ) ([^,]+) (.*) / $2 > 310 ? "$1volemd$3" : $2 == 70 ? "$1volemd1$3" : "$1$2$3" ; /exms;

(The /e flag stands for eval.) But that is much slower than split() + join().

There is no reason to use Text::CSV_XS or any other CSV module.


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Re^2: replace/substituion 4th field
by Anonymous Monk on Feb 20, 2013 at 21:40 UTC

    • There is no reason to use Text::CSV_XS or any other CSV module. For a csv file? Why is that?
    • (The /e flag stands for eval.) No sir, the /e modifier does NOT means eval.
      This is what perlop said about it; A /e will cause the replacement portion to be treated as a full-fledged Perl expression and evaluated right then and there. It is, however, syntax checked at compile-time.
      It is when you have /ee, that the second "e" evaled, the expression. Please read it up.

      There is no reason to use Text::CSV_XS or any other CSV module. For a csv file? Why is that?

      Because perl natively supplies a simple and efficient tool to do what the op wants.

      It is evaled when you have /ee, that the second "e" evaled, the expression. Please read it up.

      I originally had two ee's, but that was so slow compared to split(), I changed it to one e.

      It is, however, syntax checked at compile-time.

      ++ perl savoir-faire.

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