[ruby-core:103154] [Ruby master Bug#17771] String#start_with? should not construct MatchData or set $~

Issue #17771 has been updated by headius (Charles Nutter).


I will also point out that this method, like many others, will *not* always set $~. If you pass a string, it remains whatever it was before:

```
$ rvm ruby-3.0 do ruby -e '"foo".start_with?("foo"); p $~'
nil
```

Avoiding the use of $~ would make this behavior consistent.

----------------------------------------
Bug #17771: String#start_with? should not construct MatchData or set $~
https://bugs.ruby-lang.org/issues/17771#change-91226

* Author: headius (Charles Nutter)
* Status: Open
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
I am working on making $~ more thread-safe in JRuby and came across this unexpected behavior:

```ruby
$ rvm ruby-3.0 do ruby -e '"foo".start_with?(/foo/); p $~'
#<MatchData "foo">
```

The `start_with?` method was added 11 years ago in https://bugs.ruby-lang.org/issues/3388 but I do not think the set of $~ was an intended feature. The `start_with?` method could be much faster and more thread-safe if it did not use the frame-local backref slot and did not allocate a MatchData.

Compare with `match?` which was added specifically (without MatchData or backref setting) to provide a fast way to check if a Regexp matches.

I propose that `start_with?` stop constructing MatchData, stop setting backref, and provide only its boolean result in the same way as `match?`.



-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:[e-mail]?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>