Built In Types: String

A string is a sequence of bytes - they are not required to be valid characters in any particular encoding, for example, they may contain null bytes, or invalid UTF-8 sequences.

Basic operations

Concatenation and byte indexing are built-in operations; for example:

  • "foo"."bar" results in "foobar"
  • "abc"[1] is "b"
  • if the source code is UTF-8, "a😀c"[1] is byte 0xf0, the first of the 4 bytes compromising the "😀" emoji in UTF-8

Other operations are supported by the Str\ namespace in the Hack Standard Library, such as:

Converting to numbers

Use Str\to_int() to convert strings to integers; this will raise errors if the input contains additional data, such as .0 or other trailing characters.

(int) and (float) are more permissive, but have undefined behavior for inputs containing similar trailing data.

Bytes vs characters

Functions in the Str\ namespace with an _l suffix such as Str\length_l() take a Locale\Locale object as the first parameter, which represents the language, region/country, and encoding (e.g. en_US.UTF-8); if a multibyte encoding such as UTF-8 is specified, the _l() functions operate on characters instead of bytes. For example:

In some encodings, the same character can be represented in multiple different ways, with multiple byte sequences; the _l functions will treat them as equivalent. For example, the letter é can be represented by either:

  • "\u{00e9}", or "\xc3\xa9" ("LATIN SMALL LETTER E ACUTE")
  • "\u{0065}\u{0301}", or `\x65\xcc\x81" ("LATIN SMALL LETTER E", followed by "COMBINING ACUTE ACCENT")

This means that various comparison functions may report strings as equivalent, despite containing different byte sequences - so if the result of a character-based operation is used for another function, that function should also be character-based:

use namespace HH\Lib\{Locale,Str};

function main(): void {
  $haystack = "abc\u{00e9}"; // ends with UTF-8 "LATIN SMALL LETTER E ACUTE"
  $needle = "\u{0065}\u{0301}"; // "LATIN SMALL LETTER E", "COMBINING ACUTE ACCENT"
  $locale = Locale\create("en_US.UTF-8");
    'Byte test' => Str\ends_with($haystack, $needle), // false
    'Character test' => Str\ends_with_l($locale, $haystack, $needle), // true
    'Strip byte suffix' => Str\strip_suffix($haystack, $needle), // no change
    'Strip character suffix' => Str\strip_suffix_l($locale, $haystack, $needle), // removed
dict(4) {
  ["Byte test"]=>
  ["Character test"]=>
  ["Strip byte suffix"]=>
  string(5) "abcé"
  ["Strip character suffix"]=>
  string(3) "abc"

The Locale\bytes() locale

This locale is similar to the constant "C" or "en_US_POSIX" locale in other libraries and environments; we refer to it as the bytes locale to more clearly distinguish between this constant locale and the variable "active libc locale", and because the behavior is slightly different than libc both for performance, and to accomodate arbitrary byte sequences; for example, Str\length("a\0b") is 3, however, the libc strlen function returns 1 for the same input.

While operations such as string formatting, uppercase, and lowercase are defined for this locale, they should only be used when localization is not a concern, for example when the output is intended to be machine-readable instead of human-readable, such as when generating code.

Functions without a locale parameter

In HHVM 4.130 and above, the bytes locale is the default locale used by the Str\ functions that do not take an explicit locale - that is, Str\foo($bar) is equivalent to Str\foo_l(Locale\bytes(), $bar). In prior versions, the active native/libc locale would be used instead; Str\foo_l(Locale\get_native(), $bar) can be used to late the prior behavior.

This behavior was changed as:

  • functions such as Str\format() were frequently used to to generate machine-readable strings, leading to subtle bugs when locale was changed. For example, Str\format('%.2f', 1.23) could return either '1.23' or '1,23'.
  • functions still operated on bytes rather than characters, even with LC_CTYPE was set to UTF-8.
  • users expect functions such as Str\format() and Str\uppercase() to be pure, however they can not be when they depend on the current locale, which is effectively a global variable.


  • use the non-_l variants of functions when generating strings intended for machines to read.
  • use the non-_l variants when looking for searching for or operating on specific byte sequences.
  • avoid using setlocale() or Locale\set_native(); instead, store/pass a Locale\Locale object for the viewer in a similar way to how you store/pass other information about the viewer, such as their ID.
  • prefer Locale\set_native() over setlocale().
  • if either is necessary, restore the default locale with Locale\set_native(Locale\bytes()); while this can be functionally equivalent to various other locales (e.g. "en_US" or "C"), HHVM contains optimizations specifically for the Locale\bytes() locale.

setlocale() and Locale\set_native() affect many C libraries and extension; in web requests, this can lead to error messages in logs being translated to the viewer rather than the log reader, though for CLI programs, this behavior can be desirable.

Supported encodings

HHVM currently supports UTF-8, and single-byte encodings that are supported by the platform libc.

Other encodings may be supported in the future, however Locale\set_native() is likely to be restricted to the current locales; for example, UTF-16 can not be supported by Locale\set_native(), as UTF-16 strings can contain null bytes.