NAME¶
Scalar::String - string aspects of scalars
SYNOPSIS¶
use Scalar::String
qw(sclstr_is_upgraded sclstr_is_downgraded);
if(sclstr_is_upgraded($value)) { ...
if(sclstr_is_downgraded($value)) { ...
use Scalar::String qw(
sclstr_upgrade_inplace sclstr_upgraded
sclstr_downgrade_inplace sclstr_downgraded
);
sclstr_upgrade_inplace($value);
$value = sclstr_upgraded($value);
sclstr_downgrade_inplace($value);
$value = sclstr_downgraded($value);
DESCRIPTION¶
This module is about the string part of plain Perl scalars. A scalar has a
string value, which is notionally a sequence of Unicode codepoints, but may be
internally encoded in either ISO-8859-1 or UTF-8. In places, and more so in
older versions of Perl, the internal encoding shows through. To fully
understand Perl strings it is necessary to understand these implementation
details.
This module provides functions to classify a string by encoding and to encode a
string in a desired way.
This module is implemented in XS, with a pure Perl backup version for systems
that can't handle XS.
STRING ENCODING¶
ISO-8859-1 is a simple 8-bit character encoding, which represents the first 256
Unicode characters (codepoints 0x00 to 0xff) in one octet each. This is how
strings were historically represented in Perl. A string represented this way
is referred to as "downgraded".
UTF-8 is a variable-width character encoding, which represents all possible
Unicode codepoints in differing numbers of octets. A design feature of UTF-8
is that ASCII characters (codepoints 0x00 to 0x7f) are each represented in a
single octet, identically to their ISO-8859-1 encoding. Perl has its own
variant of UTF-8, which can handle a wider range of codepoints than Unicode
formally allows. A string represented in this variant UTF-8 is referred to as
"upgraded".
A Perl string is physically represented as a string of octets along with a flag
that says whether the string is downgraded or upgraded. At this level, to
determine the Unicode codepoints that are represented requires examining both
parts of the representation. If the string contains only ASCII characters then
the octet sequence is identical in either encoding, but Perl still maintains
an encoding flag on such a string. A string is always either downgraded or
upgraded; it is never both or neither.
When handling string input, it is good form to operate only on the Unicode
characters represented by the string, ignoring the manner in which they are
encoded. Basic string operations such as concatenation work this way (except
for a bug in perl 5.6.0), so simple code written in pure Perl is generally
safe on this front. Pieces of character-based code can pass around strings
among themselves, and always get consistent behaviour, without worrying about
the way in which the characters are encoded.
However, due to an historical accident, a lot of C code that interfaces with
Perl looks at the octets used to represent a string without also examining the
encoding flag. Such code gives inconsistent behaviour for the same character
sequence represented in the different ways. In perl 5.6, many pure Perl
operations (such as regular expression matching) also work this way, though
some of them can be induced to work correctly by using the utf8 pragma. In
perl 5.8, regular expression matching is character-based by default, but many
I/O functions (such as "open") are still octet-based.
Where code that operates on the octets of a string must be used by code that
operates on characters, the latter needs to pay attention to the encoding of
its strings. Commonly, the octet-based code expects its input to be
represented in a particular encoding, in which case the character-based code
must oblige by forcing strings to that encoding before they are passed in.
There are other usage patterns too.
You will be least confused if you think about a Perl string as a character
sequence plus an encoding flag. You should normally operate on the character
sequence and not care about the encoding flag. Occasionally you must pay
attention to the flag in addition to the characters. Unless you are writing C
code, you should try not to think about a string the other way round, as an
octet sequence plus encoding flag.
FUNCTIONS¶
Each "sclstr_" function takes one or more scalar string arguments to
operate on. These arguments must be strings; giving non-string arguments will
cause mayhem. See "is_string" in Params::Classify for a way to check
for stringness. Only the string value of the scalar is used; the numeric value
is completely ignored, so dualvars are not a problem.
Classification¶
- sclstr_is_upgraded(VALUE)
- Returns a truth value indicating whether the provided string VALUE
is in upgraded form.
- sclstr_is_downgraded(VALUE)
- Returns a truth value indicating whether the provided string VALUE
is in downgraded form.
Regrading¶
- sclstr_upgrade_inplace(VALUE)
- Modifies the string VALUE in-place, so that it is in upgraded form,
regardless of how it was encoded before. The character sequence that it
represents is unchanged.
A cleaner interface to this operation is the non-mutating
"sclstr_upgraded".
- sclstr_upgraded(VALUE)
- Returns a string that represents the same character sequence as the string
VALUE, and is in upgraded form (regardless of how VALUE is
encoded).
- sclstr_downgrade_inplace(VALUE[, FAIL_OK])
- Modifies the string VALUE in-place, so that it is in downgraded
form, regardless of how it was encoded before. The character sequence that
it represents is unchanged. If the string cannot be downgraded, because it
contains a non-ISO-8859-1 character, then by default the function
"die"s, but if FAIL_OK is present and true then it will
return leaving VALUE unmodified.
A cleaner interface to this operation is the non-mutating
"sclstr_downgraded".
- sclstr_downgraded(VALUE[, FAIL_OK])
- Returns a string that represents the same character sequence as the string
VALUE, and is in downgraded form (regardless of how VALUE is
encoded). If the string cannot be represented in downgraded form, because
it contains a non-ISO-8859-1 character, then by default the function
"die"s, but if FAIL_OK is present and true then it will
return VALUE in its original upgraded form.
SEE ALSO¶
utf8
AUTHOR¶
Andrew Main (Zefram) <zefram@fysh.org>
COPYRIGHT¶
Copyright (C) 2009, 2010, 2011 Andrew Main (Zefram) <zefram@fysh.org>
LICENSE¶
This module is free software; you can redistribute it and/or modify it under the
same terms as Perl itself.