1 | =head1 NAME |
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2 | |
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3 | perlunicode - Unicode support in Perl |
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4 | |
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5 | =head1 DESCRIPTION |
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6 | |
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7 | =head2 Important Caveat |
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8 | |
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9 | WARNING: The implementation of Unicode support in Perl is incomplete. |
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10 | |
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11 | The following areas need further work. |
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12 | |
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13 | =over |
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14 | |
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15 | =item Input and Output Disciplines |
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16 | |
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17 | There is currently no easy way to mark data read from a file or other |
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18 | external source as being utf8. This will be one of the major areas of |
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19 | focus in the near future. |
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20 | |
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21 | =item Regular Expressions |
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22 | |
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23 | The existing regular expression compiler does not produce polymorphic |
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24 | opcodes. This means that the determination on whether to match Unicode |
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25 | characters is made when the pattern is compiled, based on whether the |
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26 | pattern contains Unicode characters, and not when the matching happens |
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27 | at run time. This needs to be changed to adaptively match Unicode if |
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28 | the string to be matched is Unicode. |
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29 | |
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30 | =item C<use utf8> still needed to enable a few features |
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31 | |
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32 | The C<utf8> pragma implements the tables used for Unicode support. These |
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33 | tables are automatically loaded on demand, so the C<utf8> pragma need not |
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34 | normally be used. |
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35 | |
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36 | However, as a compatibility measure, this pragma must be explicitly used |
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37 | to enable recognition of UTF-8 encoded literals and identifiers in the |
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38 | source text. |
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39 | |
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40 | =back |
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41 | |
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42 | =head2 Byte and Character semantics |
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43 | |
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44 | Beginning with version 5.6, Perl uses logically wide characters to |
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45 | represent strings internally. This internal representation of strings |
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46 | uses the UTF-8 encoding. |
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47 | |
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48 | In future, Perl-level operations can be expected to work with characters |
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49 | rather than bytes, in general. |
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50 | |
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51 | However, as strictly an interim compatibility measure, Perl v5.6 aims to |
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52 | provide a safe migration path from byte semantics to character semantics |
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53 | for programs. For operations where Perl can unambiguously decide that the |
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54 | input data is characters, Perl now switches to character semantics. |
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55 | For operations where this determination cannot be made without additional |
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56 | information from the user, Perl decides in favor of compatibility, and |
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57 | chooses to use byte semantics. |
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58 | |
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59 | This behavior preserves compatibility with earlier versions of Perl, |
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60 | which allowed byte semantics in Perl operations, but only as long as |
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61 | none of the program's inputs are marked as being as source of Unicode |
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62 | character data. Such data may come from filehandles, from calls to |
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63 | external programs, from information provided by the system (such as %ENV), |
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64 | or from literals and constants in the source text. |
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65 | |
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66 | If the C<-C> command line switch is used, (or the ${^WIDE_SYSTEM_CALLS} |
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67 | global flag is set to C<1>), all system calls will use the |
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68 | corresponding wide character APIs. This is currently only implemented |
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69 | on Windows. |
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70 | |
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71 | Regardless of the above, the C<bytes> pragma can always be used to force |
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72 | byte semantics in a particular lexical scope. See L<bytes>. |
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73 | |
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74 | The C<utf8> pragma is primarily a compatibility device that enables |
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75 | recognition of UTF-8 in literals encountered by the parser. It may also |
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76 | be used for enabling some of the more experimental Unicode support features. |
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77 | Note that this pragma is only required until a future version of Perl |
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78 | in which character semantics will become the default. This pragma may |
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79 | then become a no-op. See L<utf8>. |
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80 | |
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81 | Unless mentioned otherwise, Perl operators will use character semantics |
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82 | when they are dealing with Unicode data, and byte semantics otherwise. |
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83 | Thus, character semantics for these operations apply transparently; if |
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84 | the input data came from a Unicode source (for example, by adding a |
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85 | character encoding discipline to the filehandle whence it came, or a |
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86 | literal UTF-8 string constant in the program), character semantics |
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87 | apply; otherwise, byte semantics are in effect. To force byte semantics |
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88 | on Unicode data, the C<bytes> pragma should be used. |
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89 | |
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90 | Under character semantics, many operations that formerly operated on |
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91 | bytes change to operating on characters. For ASCII data this makes |
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92 | no difference, because UTF-8 stores ASCII in single bytes, but for |
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93 | any character greater than C<chr(127)>, the character may be stored in |
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94 | a sequence of two or more bytes, all of which have the high bit set. |
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95 | But by and large, the user need not worry about this, because Perl |
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96 | hides it from the user. A character in Perl is logically just a number |
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97 | ranging from 0 to 2**32 or so. Larger characters encode to longer |
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98 | sequences of bytes internally, but again, this is just an internal |
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99 | detail which is hidden at the Perl level. |
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100 | |
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101 | =head2 Effects of character semantics |
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102 | |
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103 | Character semantics have the following effects: |
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104 | |
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105 | =over 4 |
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106 | |
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107 | =item * |
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108 | |
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109 | Strings and patterns may contain characters that have an ordinal value |
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110 | larger than 255. |
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111 | |
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112 | Presuming you use a Unicode editor to edit your program, such characters |
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113 | will typically occur directly within the literal strings as UTF-8 |
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114 | characters, but you can also specify a particular character with an |
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115 | extension of the C<\x> notation. UTF-8 characters are specified by |
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116 | putting the hexadecimal code within curlies after the C<\x>. For instance, |
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117 | a Unicode smiley face is C<\x{263A}>. A character in the Latin-1 range |
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118 | (128..255) should be written C<\x{ab}> rather than C<\xab>, since the |
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119 | former will turn into a two-byte UTF-8 code, while the latter will |
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120 | continue to be interpreted as generating a 8-bit byte rather than a |
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121 | character. In fact, if the C<use warnings> pragma of the C<-w> switch |
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122 | is turned on, it will produce a warning |
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123 | that you might be generating invalid UTF-8. |
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124 | |
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125 | =item * |
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126 | |
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127 | Identifiers within the Perl script may contain Unicode alphanumeric |
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128 | characters, including ideographs. (You are currently on your own when |
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129 | it comes to using the canonical forms of characters--Perl doesn't (yet) |
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130 | attempt to canonicalize variable names for you.) |
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131 | |
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132 | =item * |
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133 | |
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134 | Regular expressions match characters instead of bytes. For instance, |
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135 | "." matches a character instead of a byte. (However, the C<\C> pattern |
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136 | is provided to force a match a single byte ("C<char>" in C, hence |
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137 | C<\C>).) |
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138 | |
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139 | =item * |
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140 | |
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141 | Character classes in regular expressions match characters instead of |
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142 | bytes, and match against the character properties specified in the |
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143 | Unicode properties database. So C<\w> can be used to match an ideograph, |
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144 | for instance. |
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145 | |
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146 | =item * |
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147 | |
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148 | Named Unicode properties and block ranges make be used as character |
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149 | classes via the new C<\p{}> (matches property) and C<\P{}> (doesn't |
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150 | match property) constructs. For instance, C<\p{Lu}> matches any |
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151 | character with the Unicode uppercase property, while C<\p{M}> matches |
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152 | any mark character. Single letter properties may omit the brackets, so |
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153 | that can be written C<\pM> also. Many predefined character classes are |
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154 | available, such as C<\p{IsMirrored}> and C<\p{InTibetan}>. |
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155 | |
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156 | =item * |
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157 | |
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158 | The special pattern C<\X> match matches any extended Unicode sequence |
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159 | (a "combining character sequence" in Standardese), where the first |
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160 | character is a base character and subsequent characters are mark |
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161 | characters that apply to the base character. It is equivalent to |
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162 | C<(?:\PM\pM*)>. |
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163 | |
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164 | =item * |
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165 | |
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166 | The C<tr///> operator translates characters instead of bytes. It can also |
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167 | be forced to translate between 8-bit codes and UTF-8. For instance, if you |
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168 | know your input in Latin-1, you can say: |
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169 | |
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170 | while (<>) { |
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171 | tr/\0-\xff//CU; # latin1 char to utf8 |
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172 | ... |
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173 | } |
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174 | |
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175 | Similarly you could translate your output with |
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176 | |
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177 | tr/\0-\x{ff}//UC; # utf8 to latin1 char |
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178 | |
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179 | No, C<s///> doesn't take /U or /C (yet?). |
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180 | |
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181 | =item * |
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182 | |
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183 | Case translation operators use the Unicode case translation tables |
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184 | when provided character input. Note that C<uc()> translates to |
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185 | uppercase, while C<ucfirst> translates to titlecase (for languages |
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186 | that make the distinction). Naturally the corresponding backslash |
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187 | sequences have the same semantics. |
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188 | |
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189 | =item * |
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190 | |
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191 | Most operators that deal with positions or lengths in the string will |
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192 | automatically switch to using character positions, including C<chop()>, |
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193 | C<substr()>, C<pos()>, C<index()>, C<rindex()>, C<sprintf()>, |
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194 | C<write()>, and C<length()>. Operators that specifically don't switch |
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195 | include C<vec()>, C<pack()>, and C<unpack()>. Operators that really |
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196 | don't care include C<chomp()>, as well as any other operator that |
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197 | treats a string as a bucket of bits, such as C<sort()>, and the |
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198 | operators dealing with filenames. |
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199 | |
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200 | =item * |
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201 | |
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202 | The C<pack()>/C<unpack()> letters "C<c>" and "C<C>" do I<not> change, |
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203 | since they're often used for byte-oriented formats. (Again, think |
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204 | "C<char>" in the C language.) However, there is a new "C<U>" specifier |
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205 | that will convert between UTF-8 characters and integers. (It works |
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206 | outside of the utf8 pragma too.) |
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207 | |
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208 | =item * |
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209 | |
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210 | The C<chr()> and C<ord()> functions work on characters. This is like |
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211 | C<pack("U")> and C<unpack("U")>, not like C<pack("C")> and |
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212 | C<unpack("C")>. In fact, the latter are how you now emulate |
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213 | byte-oriented C<chr()> and C<ord()> under utf8. |
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214 | |
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215 | =item * |
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216 | |
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217 | And finally, C<scalar reverse()> reverses by character rather than by byte. |
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218 | |
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219 | =back |
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220 | |
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221 | =head2 Character encodings for input and output |
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222 | |
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223 | [XXX: This feature is not yet implemented.] |
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224 | |
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225 | =head1 CAVEATS |
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226 | |
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227 | As of yet, there is no method for automatically coercing input and |
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228 | output to some encoding other than UTF-8. This is planned in the near |
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229 | future, however. |
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230 | |
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231 | Whether an arbitrary piece of data will be treated as "characters" or |
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232 | "bytes" by internal operations cannot be divined at the current time. |
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233 | |
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234 | Use of locales with utf8 may lead to odd results. Currently there is |
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235 | some attempt to apply 8-bit locale info to characters in the range |
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236 | 0..255, but this is demonstrably incorrect for locales that use |
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237 | characters above that range (when mapped into Unicode). It will also |
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238 | tend to run slower. Avoidance of locales is strongly encouraged. |
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239 | |
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240 | =head1 SEE ALSO |
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241 | |
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242 | L<bytes>, L<utf8>, L<perlvar/"${^WIDE_SYSTEM_CALLS}"> |
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243 | |
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244 | =cut |
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