in reply to Re: Progressive matching w/substitutions
in thread Progressive matching w/substitutions

You said:
Finally, this is really the task of the "e" modifier.
$string =~ s/(abc)/ sprintf("%s def(%s)", $1, $x++) /eig;
Now you're getting directly to my point. Let's now rewrite this to using a while loop, as in:

while ($string =~ s/(abc)/  sprintf("%s def(%s)", $1, $x)  /eig) {
    die "oops" if $x++ > 10;
}

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Why doesn't this work? If you remove the 'g' modifier, it still doesn't work -- it just runs in an infinite loop.

I think what I'm getting to is that I assumed that the "progressive" aspect of regex applied to both matching and substitution, since each of those has the common root of matching. (ie., substitution requires at least a match in order to do the substitution) If that were the case, then the above should work. Because it doesn't, the code has to be downgraded to using multiple lines that include the use of pos and such. In the live code where this applies, calls to other functions have to be made, which is why there has to be a loop, not just a simple s/// expression as your example illustrates. Granted, the "substitution" aspect can be as simple as you have, but the value of $x is derived from other code and various tests and manipulations. That's why my "simple" example uses $x++ inside the while loop, to illustrate that it has to be separate from the s/// part.

Dan Heller
http://www.danheller.com

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Re^3: Progressive matching w/substitutions
by ikegami (Patriarch) on Aug 09, 2008 at 17:18 UTC

    Why doesn't this work?

    Read up on the return value of s/// in scalar context.

    calls to other functions have to be made

    What do you think sprintf is? The body of the loop is the replace expression. Put the die in there.

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      Why doesn't this work?
      Read up on the return value of s/// in scalar context.
      That doesn't apply. both m// and s/// return values that can be interpreted by a while() construct to evaluate to a boolean. The point isn't what the s/// expression returns, it's whether matching (then substitution) ia done "progressively" as the documentation says it would for m// in the context of a while() loop. The fact that the documentation doesn't say anything about s/// (at least, not in the ORA perl book) leads me to consider it to be either an oversight in implementation, or an outright bug. :-/
      calls to other functions have to be made
      What do you think sprintf is? The body of the loop is the replace expression. Put the die in there.
      No, that would be involved and overly obfuscated. Granted, it can be done, and there's no question that perl is robust enough that you could implement any solution using its existing implementation. but just because there's "another way to do it" is not a good reason to always defend an aspect to the current design that may be potentially... well, "undefined".

      I'm not trying to be argumentative; I'm only trying to focus on this one aspect of s/// and how "progressive matching" has been defined/implemented. I appreciate the explanations for how ELSE to accomplish the task, but that's not what I'm hoping to discuss.

      Dan Heller
      http://www.danheller.com
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        Modifying a string's value resets pos.

        - tye        

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