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.