From: "mame (Yusuke Endoh) via ruby-core" <ruby-core@...>
Date: 2023-08-24T11:48:20+00:00
Subject: [ruby-core:114486] [Ruby master Bug#19784] String#delete_prefix! problem

Issue #19784 has been updated by mame (Yusuke Endoh).





Discussed at the dev meeting.

We agreed that both `start_with?` and `delete_prefix` should compare character by character if valid, and byte by byte for invalid.



```ruby

"\xFF\xFE".start_with?("\xFF")   #=> true   # not changed

"\xFF\xFE".delete_prefix("\xFF") #=> "\xFE" # changed

"�".start_with?("\xC3")          #=> false  # changed

"�".delete_prefix("\xC3")        #=> "�"    # not changed

```



Note that encoding validity is checked on a position-by-position basis:



```ruby

"\xFF�".delete_prefix("\xFF") #=> should be "�"

"\xFF�".delete_prefix("\xFF\xC3") #=> should be "\xFF�"

"\xFF�".delete_prefix("\xFF\xC3\x84") #=> should be ""

```



----------------------------------------

Bug #19784: String#delete_prefix! problem

https://bugs.ruby-lang.org/issues/19784#change-104272



* Author: inversion (Yura Babak)

* Status: Open

* Priority: Normal

* ruby -v: ruby 3.2.2 (2023-03-30 revision e51014f9c0) [x86_64-linux]

* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN

----------------------------------------

Here is the snipped and the question is in the comments:

``` ruby

fp = 'with_BOM_16.txt'

body = File.read(fp).force_encoding('UTF-8')



p body                            # "\xFF\xFE1\u00001\u0000"

p body.start_with?("\xFF\xFE")    # true



body.delete_prefix!("\xFF\xFE")   # !!! why doesn't work?

p body                            # "\xFF\xFE1\u00001\u0000"

p body.start_with?("\xFF\xFE")    # true



body[0, 2] = ''

p body                            # "1\u00001\u0000"

p body.start_with?("\xFF\xFE")    # false



```

Works same 

on Linux (ruby 3.2.2 (2023-03-30 revision e51014f9c0) [x86_64-linux])

and Windows (ruby 3.2.2 (2023-03-30 revision e51014f9c0) [x64-mingw-ucrt])







-- 

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