Bug #7092

StringScanner start-of-line matches false positives, while lookbehind for a newline gives false negatives

Added by Jay Adkisson over 4 years ago. Updated over 3 years ago.

Target version:
ruby -v:
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-linux]


To reproduce:

>> require 'strscan'
>> ss ="ab")
>> ss.scan(/./)
=> "a"
>> ss.scan(/^./) # expecting nil, since the head is in the middle of a line
=> "b"

As a workaround-that-didn't-work, I also tried looking behind for a newline, which seems never to match, even with the m flag.

>> ss ="a\nb")
>> ss.scan(/../m)
=> "a\n"
>> ss.scan(/(?<=\n)./m) # expecting "b", since the previous char is a newline
=> nil


#1 [ruby-core:48143] Updated by Jay Adkisson over 4 years ago

Bump. Can someone confirm whether or not this is expected behavior?

#2 [ruby-core:48756] Updated by Jay Adkisson over 4 years ago

I've now reproduced this in 2.0.0-preview1. Is anyone out there...?

#3 [ruby-core:48758] Updated by Yusuke Endoh over 4 years ago

  • Status changed from Open to Feedback
  • Assignee set to Yusuke Endoh
  • Target version set to 2.0.0

Sorry for late reply.

Honestly, it is difficult to say whether it is intended or a bug because there is no strscan maintainer.
But since 1.8, StringScanner#scan has behaved so. I guess there is some existing programs that depends on the behavior.

Therefore, I'd like not to change the behavior unless there is no special reason.
Do you have any special reason? Or do you just want to confirm?

Yusuke Endoh

#4 [ruby-core:48759] Updated by Yusuke Endoh over 4 years ago

FYI: You may want to use StringScanner#beginning_of_line?.

Yusuke Endoh

#5 [ruby-core:48761] Updated by Nobuyoshi Nakada over 4 years ago

I guess it would be intentional.
'' and '\A' are `beginning of the target' (and beginning of a line for ) anchors, so seems reasonable that it matches the beginning of the part to be matched.

#6 [ruby-core:52038] Updated by Yusuke Endoh about 4 years ago

  • Priority changed from Normal to 3
  • Target version changed from 2.0.0 to next minor

#7 [ruby-core:55215] Updated by Yusuke Endoh over 3 years ago

  • Status changed from Feedback to Rejected

I'm marking this ticket as closed due to no feedback.

Yusuke Endoh

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