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Re: simple regex pattern sought

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On 5/26/2012 6:19 AM, Roedy Green wrote:

>          exercisePattern( Pattern.compile(
> "\"(?:\\\\.|[^\\\"])*\"|'(?:\\\\.|[^\\'])*'" ) ); // works, accepts
> empty strings
>          // (?: ) is a non-capturing group. This is Robert Klemme's
> contribution. I don't understand how it works.


Ah, OK, so here's my contribution to your excellent SSCCE.  First this 
pattern is basically the same as mine.  It uses alternation (the 
vertical bar |) to pick a string delimited by either ' or "

Here's his regex string without the extra escapes for Java:

"(?:\\.|[^\"])*"|'(?:\\.|[^\'])*'
^^^^^^^^^^^^^^^^

Let's look at just the first half for a moment, without the (?:\\. part.

        "[^\"]*"
        ^^^^^^^^
        12     3
Example for the first part:
   1. "        string starts with double quote
   2. [^\"]*   doesn't contain a "
   3. "        ends with double quote

Same for the second half of the string.

Notice he's using * instead of +'s, which is why his matches 0 width 
strings.

The other part didn't appear in your problem statement, but in HTML/XML 
it's allowed to escape characters.  E.g., 'Bob\'s your uncle.'  So his 
inclusion is very reasonable.

So he Robert adds (\\.|[^\"])* to the first part, which is
                   12 345     6

1. Start a group
2. A slash.  It needs to be escaped for regex, hence \\.
3. . is regex "any character".  2 and 3 together mean "match \ followed 
by any character"
4. OR (alternation again)
5. character class, negated (the ^), matches anything except \ or ".  I 
think this is a mistake:  the \ needs to be quoted.
6. zero or more.

Then after that mess, he does the obvious thing and adds non-capturing 
group, to make the regex do a little less work.

   "(?:\\.|[^\"])*"

Phew!  Next, he adds one alternation and does the same for a ' delimited 
string.

|'(?:\\.|[^\'])*'

Same thing, just ' instead of ".

Finally I think this could be simplified slightly with Lew's 
back-reference idea.

(['"])(?:\\.|[^\1\\])*

(Untested.)  This allows empty strings between delimiters;  instead of a 
* use + for only non-empty strings between the quotes.



My executive summary:

Regex is a great rapid development tool, except when it isn't.  You 
realize your problem is simple, and you could have hand-coded a parser 
to do this much quicker than all these news post exchanges?

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Thread

simple regex pattern sought Roedy Green <see_website@mindprod.com.invalid> - 2012-05-25 14:45 -0700
  Re: simple regex pattern sought markspace <-@.> - 2012-05-25 14:55 -0700
  Re: simple regex pattern sought Lew <lewbloch@gmail.com> - 2012-05-25 14:55 -0700
    Re: simple regex pattern sought markspace <-@.> - 2012-05-25 15:04 -0700
      Re: simple regex pattern sought Lew <noone@lewscanon.com> - 2012-05-26 14:07 -0700
        Re: simple regex pattern sought markspace <-@.> - 2012-05-26 18:34 -0700
          Re: simple regex pattern sought Lew <noone@lewscanon.com> - 2012-05-27 11:39 -0700
    Re: simple regex pattern sought Lew <lewbloch@gmail.com> - 2012-05-25 15:03 -0700
    Re: simple regex pattern sought Robert Klemme <shortcutter@googlemail.com> - 2012-05-26 00:12 +0200
      Re: simple regex pattern sought markspace <-@.> - 2012-05-25 18:43 -0700
        Re: simple regex pattern sought Robert Klemme <shortcutter@googlemail.com> - 2012-05-26 16:37 +0200
          Re: simple regex pattern sought markspace <-@.> - 2012-05-26 08:06 -0700
            Re: simple regex pattern sought Robert Klemme <shortcutter@googlemail.com> - 2012-05-26 17:34 +0200
              Re: simple regex pattern sought Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-05-26 10:07 -0700
      Re: simple regex pattern sought Roedy Green <see_website@mindprod.com.invalid> - 2012-05-26 06:19 -0700
        Re: simple regex pattern sought markspace <-@.> - 2012-05-26 07:19 -0700
        Re: simple regex pattern sought markspace <-@.> - 2012-05-26 07:57 -0700
          Re: simple regex pattern sought Robert Klemme <shortcutter@googlemail.com> - 2012-05-26 17:13 +0200
            Re: simple regex pattern sought markspace <-@.> - 2012-05-26 10:08 -0700
              Re: simple regex pattern sought Roedy Green <see_website@mindprod.com.invalid> - 2012-05-26 14:14 -0700

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