Timjc86 has asked for the wisdom of the Perl Monks concerning the following question:

Hello, monks! I am starting my third week of Perl experience. Coming from a (limited) C++ background, I am fascinated with the ease and potency of regular expressions. But now I have run into something I don't think regexes can solve.

The problem: I have a number of strings composed of a hodgepodge of information - dates, times, comments, other numbers, etc. I am trying to replace some of the dates with their epoch timestamps.

Thus far:

Where I'm stuck: I'm not sure how to get the timestamp back into the appropriate string in the proper place(s). I have tried using a subroutine with search and replace, i.e. s/<regex to find date>/&<subroutine to calculate date's timestamp>/ge, but I continue to get syntax errors (I assume because you can't call subroutines inside a regex).

Thoughts on how to proceed:

  1. This would be easily solved if I could find essentially a "search, modify, replace" function or something similar. I'm only assuming one cannot call subroutines inside s/ / / because I am getting errors trying to do so - if it is indeed possible, please correct me - and maybe provide a dummy example too please. ^_^
  2. I guess I could also completely rebuild the strings, i.e. breakup every field (into an array), modify the ones I need to, and then reassemble the string. It just seems tedious and messy. I wanted to check with some more experienced folks to make sure I wasn't doing a lot of silly and unnecessary work.

Thanks!

Replies are listed 'Best First'.
Re: Search, modify, replace?
by ikegami (Patriarch) on Jun 10, 2009 at 14:59 UTC

    You want to make changes, so you want the substitution operator (s///). And you want the replacement to be the result of a Perl expression, so you need to use the "e" modifier. 

    use Time::Local qw( timelocal );

s/(\d{4})-(\d{2})-(\d{2})/ timelocal(0,0,0, $3,$2-1,$1) /ge;

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    but I continue to get syntax errors (I assume because you can't call subroutines inside a regex).

    Maybe there's something wrong with the bits you omitted (like an unescaped "/"?) since the bits you did post are fine. Next time, please post the actual code and error messages.

    Well, except that you're using "&" on your function call. Why are you telling Perl to ignore prototypes?

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Re: Search, modify, replace?
by JavaFan (Canon) on Jun 10, 2009 at 15:03 UTC
    I'm not sure how to get the timestamp back into the appropriate string in the proper place(s). I have tried using a subroutine with search and replace, i.e.s/<regex to find date>/&<subroutine to calculate date's timestamp>/ge, but I continue to get syntax errors (I assume because you can't call subroutines inside a regex).
    You don't have the subroutine inside the regex, and you don't have to. Your approach is sound. So the syntax error must be in the details. Which you did't post.
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Re: Search, modify, replace?
by Timjc86 (Novice) on Jun 10, 2009 at 15:28 UTC

    Thank you all for your speedy replies, I really appreciate it.

    ikegami, that looks like exactly what I was looking for, thank you. I'm not sure why I never realized I could directly call timelocal() inside s///. I also did not properly understand the $1, $2, etc. special variables, but that example illustrates their use perfectly.

    Well, except that you're using "&" on your function call. Why are you telling Perl to ignore prototypes?
    The "&" was my attempt to call a subroutine inside the s///. My thought process was to have the subroutine do the work of appropriately arranging the date elements (day, month, year, etc), calling timelocal(), and returning the epoch timestamp. I have a knack for over-complicating my own code. =) I'm also still learning with regexes in general - I don't yet remember all of the characters that have special meanings inside a regex (as it sounds like "&" does: to ignore prototypes?).

    I will give this a shot. Thanks again everyone.

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      I don't yet remember all of the characters that have special meanings inside a regex (as it sounds like "&" does: to ignore prototypes?).
      The  & (ampersand) character is not a regular expression metacharacter, i.e., it has no special meaning in a regex.

      The  & sigil does have a special meaning with respect to subroutine invocation. See perlsub and search for '&', especially in the first part of the DESCRIPTION section.

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Re: Search, modify, replace?
by Utilitarian (Vicar) on Jun 10, 2009 at 15:02 UTC
    Never mind, forgot about the e modifier
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