PHP: rfc:negative-string-offsets

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====== PHP RFC: Generalize support of negative string offsets ======
  * Version: 1.3
  * Date: 2016-02-18
  * Author: François Laupretre <francois@php.net>
  * Status: Implemented (in PHP 7.1)
  * First Published at: http://wiki.php.net/rfc/negative-string-offsets

===== Introduction =====

In most PHP functions, providing a negative value as string offset means '//n// positions counted backwards from the end of the string'.
This mechanism is widely used but, unfortunately, these negative values are not supported everywhere this would make sense.
So, as PHP developers cannot easily know whether a given string functions accepts negative values or not,
they regularly need to refer to the documentation.
If it does not, they need to insert a call to the substr() function, making their code less readable and slower.

An obvious example is strrpos() accepting negative offsets, while strpos() does not.
The same with substr_count() rejecting negative offset or length, when substr() accepts them.

===== Proposal =====

This RFC proposes to generalize support for negative string offsets everywhere it makes sense.

In accordance with the existing behavior, a negative offset is a position counted backwards from the end of the string.

In the same spirit, the RFC also adds support for negative length arguments where it makes sense.
A negative length (-//x//) means 'up to the position //x// counted backwards from the end of the string'.

The reference behavior for negative offset and length is the [[http://php.net/manual/en/function.substr.php|substr()]] function.

The main objective of this RFC is to improve the overall consistency of the language.

==== Feature requests solved by this RFC ====

  * https://bugs.php.net/bug.php?id=50649
  * https://bugs.php.net/bug.php?id=36524

===== Detailed changes =====

==== In the language ====

String access to individual characters using a '{}' or '[]' will be extended to negative values.

Examples :

<code php>
$str='abcdef';
var_dump($str[-2]); // => string(1) "e"

$str{-3}='.';
var_dump($str);		// => string(6) "abc.ef"

var_dump(isset($str{-4}));	// => bool(true)

var_dump(isset($str{-10}));	// => bool(false)
</code>

==== In built-in functions ====

^ Function ^ Add support for ^
| strpos | Negative offset |
| stripos | Negative offset |
| substr_count| Negative 'offset' and 'length' (same behavior as substr()) |
| grapheme_strpos | Negative offset |
| grapheme_stripos | Negative offset |
| grapheme_extract | Negative offset |
| iconv_strpos | Negative offset |
| file_get_contents| Negative offset (based on seek(SEEK_END), reserved to seekable streams) |
| mb_strimwidth | Negative 'start' and 'width' |
| mb_ereg_search_setpos| Negative 'position' when search string is defined |
| mb_strpos| Negative offset |
| mb_stripos| Negative offset |

==== Notes ====

  * Nothing done for iconv_strrpos() because function does not accept an 'offset' arg (inconsistent with every other xxx_strrpos() functions but argument cannot be added without breaking BC).

  * file_get_contents() : 'maxlen' argument cannot support negative values because of stream filters.

===== Backward Incompatible Changes =====

This RFC extends the range of valid values.
In most cases, negative values raise a warning message and offset is considered as zero.
The new behavior considers such values as valid (as long as they don't exceed the string length).

While not negligible, I consider these BC breaks as minor because, everywhere the behavior is modified, negative values were considered as invalid and raised an error message.
So, we are just suppressing error cases.

===== Proposed PHP Version(s) =====

7.1

===== RFC Impact =====
==== To SAPIs ====

None

==== To Existing Extensions ====

None

==== To Opcache ====

None

===== Open Issues =====

To do (waiting for RFC approval) :

  * Update documentation
  * Update language specifications

===== Unaffected PHP Functionality =====

mbstring functions remain compatible with their ASCII counterpart, relative to the mbstring.func_overload ini setting.

===== Future Scope =====

==== Recommend using '{}' vs '[]' for string offsets ====

It was suggested during the discussion that, since array access and string
offsets are very different operations, the official documentation should
recommend using the '{}' syntax for string offsets, instead of the ambiguous
'[]' syntax (and potentially deprecate '[]' for string offsets in the future).

On the opposite side, it was also suggested that array access and string offsets
are so closely-related concepts that we should recommend using '[]' in both
cases and disable the alternate '{}' syntax for string offsets !

So, as the subject is controversial and very tangential to the subject of this RFC,
it will be left for a future RFC.

===== Proposed Voting Choices =====

As this RFC adds support for negative string offsets in the language, it requires a 2/3 majority.

<doodle title="Generalize support of negative string offsets" auth="francois" voteType="single" closed="true">
   * Yes
   * No
</doodle>

===== Patches and Tests =====

Pull Request : [[https://github.com/php/php-src/pull/1431]] (final patch)

===== Implementation =====
After the project is implemented, this section should contain 
  - the version(s) it was merged to
  - a link to the git commit(s)
  - a link to the PHP manual entry for the feature

===== References =====
None

===== Rejected Features =====
Keep this updated with features that were discussed on the mail lists.
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