Bug #7442


StringScanner#charpos vs StringScanner#pos

Added by zenspider (Ryan Davis) almost 10 years ago. Updated almost 10 years ago.

Target version:
ruby -v:


I talked to Matz at rubyconf and he agreed this was a bug I should file. Sorry I took so long to do so.

As mentioned in #3482, StringScanner#pos is byte-oriented even when scanning multibyte strings. The reasoning was that IO#pos is byte-oriented so this is to spec and functioning correctly. The problem is that StringScanner isn't just an IO as it also represents a String and the progress scanning through it. Strings in 1.9+ must respect their encodings and with a few exceptions don't even support the idea of naked bytes. I think StringScanner must be able to respect that.

Given that ss is a StringScanner instance on a string with a valid encoding, getting the substring of the current progress via ss.string[] can result in a String with invalid encoding. I propose that we add #charpos to make it possible to pull out a valid substring. This would also be useful towards being able to report proper offset or column information in the case of an error when you're using StringScanner as your lexer.

This is the code that I needed to get proper char-offsets (and substrings--I needed both for my purposes):

def string_to_pos
  string.byteslice(0, pos)

def charpos


Related issues 1 (0 open1 closed)

Related to Ruby master - Feature #1159: StringScanner に文字ベースでのインデックスを返すメソッドがほしいRejectedaamine (Minero Aoki)02/14/2009Actions

Updated by mame (Yusuke Endoh) almost 10 years ago

  • Status changed from Open to Feedback
  • Target version set to 3.0

Sorry, it is too late to fix such a spec-level bug. Setting the target to Next Major.
If you create and commit a patch by preview2 (1 Dec.), and if it does not lead to any problem (and any discussion) at all, we might include it in 2.0.0.

Yusuke Endoh

Updated by zenspider (Ryan Davis) almost 10 years ago

Committed revision 37916.

Please beat up on it.

Updated by zenspider (Ryan Davis) almost 10 years ago

No objections (yet)... can this be merged to 2.0 branch for next preview release?

Updated by mame (Yusuke Endoh) almost 10 years ago

  • Status changed from Feedback to Closed

I think so. We can keep it unless any serious problem is reported after preview2. Thanks for your quick action!

I'm slightly worried about its very inefficient implementation, but I don't know whether it matters because I understand the use case. Anyway, we can refine it after 2.0.0.

Yusuke Endoh


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