Re^3: Regex conditional match if previous match in same expression is true?

can you think of a solution that uses the conditional notation

The algorithm:

Note the preceding character (if any).
Match 'hello'.
If there was a character to note,
- If the noted character is {,
  - Match }. ( Oops, I had the brace reversed. )
- Else,
  - Negatively match }.
Else,
- Negatively match }.

As it turns out, the "if any" portion of the first step is hard to implement because look-behinds must be fixed-width. So let's make that conditional too:

If we are at the start of the string,
- Match 'hello'.
- Negatively match }.
Else,
- Note the preceding character.
- Match 'hello'.
- If the noted character is {,
  - Match }. ( Oops, I had the brace reversed. )
- Else,
  - Negatively match }.

/
   (?(?<=(.))
      # We are not at the start of the string.
      # The preceding character is in $1.
      hello
      (?(?{ $1 eq "\{" })
         # The char before 'hello' is '{'.
         }
      |
         # The char before 'hello' is not '{'.
         (?! } )
      )
   |
      # We are at the start of the string.
      hello
      (?! } )
   )
/x
[download]

Yikes!

Comment on Re^3: Regex conditional match if previous match in same expression is true? Select or Download Code

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Re^4: Regex conditional match if previous match in same expression is true? by radiantmatrix (Parson) on Apr 10, 2007 at 15:55 UTC
Thank you! That was deeply instructive -- where I work I code primarily in a vacuum (well, there is one other Perl user, but he's just now learning), so it's nice to see the thought process of a more experienced Monk. Just out of curiosity, I note that you used the Perl-only capability of executing Perl code inside a conditional. Despite being way off topic, I am interested to know if that's really the only sensible way to do this (that is, accomplish my goal using a conditional rather than an alternation). For example, if I were to use Jakarta-ORO (Perl-style regex engine for Java), I suspect I wouldn't have that facility. Feel free to /msg me if you don't want to carry this OT discussion in-thread, we could pick it up in e-mail. Or, if you prefer, could you recommend a chapter of Mastering Regular Expressions that covers the topic well? I've bought the book, but haven't yet had the time to read it start-to-finish. <–radiant.matrix–> Ramblings and references The Code that can be seen is not the true Code I haven't found a problem yet that can't be solved by a well-placed trebuchet	[reply]
Re^5: Regex conditional match if previous match in same expression is true? by ikegami (Patriarch) on Apr 10, 2007 at 16:23 UTC
Rhandom provided a better solution. Here's a simplification of it: `/ (?: ({) \| (?<!{) ) hello (?(1) } \| (?!}) ) /x` [download] It doesn't "use Perl", but I suspect it won't solve your compatibility problems like you think it will. Are you sure that engine supports conditionals (`(?(...)...\|...)`)? I've never read "Mastering Regular Expressions".	[reply] [d/l] [select]
Re^6: Regex conditional match if previous match in same expression is true? by radiantmatrix (Parson) on Apr 23, 2007 at 14:25 UTC
Yes, the Jakarta-ORO provides a `Perl5Matcher` class (and relatives) that supports all of the Perl-5 regex engine, with the (fairly obvious) exception of being able to execute Perl inside a regex. It's been a lifesaver on one of my current projects, where I have to use Java (plugins for a J2EE app, plus boss requirement) to do tasks particularly suited to regexen. I'm thankful to the Jakarta folks for making it so that I don't have to learn Yet Another Regex Syntax. ;-) <–radiant.matrix–> Ramblings and references The Code that can be seen is not the true Code I haven't found a problem yet that can't be solved by a well-placed trebuchet	[reply] [d/l]