[ruby-core:114278] [Ruby master Bug#19784] String#delete_prefix! problem

Issue #19784 has been updated by byroot (Jean Boussier).


I suspect it's because `"\xFF\xFE1\u00001\u0000" (UTF-8)` has invalid encoding.

That said, if `starts_with?` returns true, `delete_prefix` should arguably work.

I'll try to see if I can find why, but in the meantime I tested that you can workaround this by casting the string to `Encoding::BINARY`:

```ruby
str = "\xff\xfe1\u00001\u0000"
str.force_encoding(Encoding::BINARY)
str.delete_prefix!("\xff\xfe".b)
str.force_encoding(Encoding::UTF_8)
p str # => "1\u00001\u0000"
```



----------------------------------------
Bug #19784: String#delete_prefix! problem
https://bugs.ruby-lang.org/issues/19784#change-103973

* 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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