[ruby-core:92995] [Ruby trunk Feature#15899] String#before and String#after

Issue #15899 has been updated by kke (Kimmo Lehto).


> Using partition looks reasonable, and it can accept regexes.

It also has the problem of creating extra objects that you need to discard with `_` or assign and just leave unused.

> I am not a big fan of the names, though. I somehow associate #before and #after
more with time-based operations; and rack/sinatra middleware (route) filters.

How about `str.preceding(';')` and `str.following(';')`? 

Perhaps `str.prior_to(';')` and `str.behind(';')`?

Possibility of opposite reading direction can make these problematic.

`str.left_from(';')`, `str.right_from(';')`? Sounds a bit clunky.

Head and tail could be the unixy choice and more versatile for other use cases.

```ruby
class String
  def head(count = 10, separator = "\n")
    ...
  end

  def tail(count = 10, separator = "\n")
    ...
  end
end
```

For my example use case, it would become:


```ruby
str = "application/json; charset=utf-8"
mime = str.head(1, ';')
labels = str.tail(1, ';')
```

And to emulate something like `$ curl xttp://x.example.com | head` you would use `response.body.head`


----------------------------------------
Feature #15899: String#before and String#after
https://bugs.ruby-lang.org/issues/15899#change-78373

* Author: kke (Kimmo Lehto)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
There  seems to be no methods for getting a substring before or after a marker.

Too often I see and have to resort to variations of:

``` ruby
str[/(.+?);/, 1]
str.split(';').first
substr, _ = str.split(';', 2)
str.sub(/.*;/, '')
str[0...str.index(';')]
```

These create intermediate objects or/and are ugly.

The `String#delete_suffix` and `String#delete_prefix` do not accept regexps and thus only can be used if you first figure out the full prefix or suffix.

For this reason, I suggest something like:

``` ruby
> str = 'application/json; charset=utf-8'
> str.before(';')
=> "application/json"
> str.after(';')
=> " charset=utf-8"
```

What should happen if the marker isn't found? In my opinion, `before` should return the full string and `after` an empty string. 





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