1String::Base(3pm) User Contributed Perl Documentation String::Base(3pm)
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6 String::Base - string index offseting
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9 use String::Base +1;
10 no String::Base;
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14 This module implements automatic offsetting of string indices. In
15 normal Perl, the first character of a string has index 0, the second
16 character has index 1, and so on. This module allows string indexes to
17 start at some other value. Most commonly it is used to give the first
18 character of a string the index 1 (and the second 2, and so on), to
19 imitate the indexing behaviour of FORTRAN and many other languages. It
20 is usually considered poor style to do this.
21 The string index offset is controlled at compile time, in a lexically-
23 scoped manner. Each block of code, therefore, is subject to a fixed
24 offset. It is expected that the affected code is written with
25 knowledge of what that offset is.
26 Using a string index offset
28 A string index offset is set up by a "use String::Base" directive, with
29 the desired offset specified as an argument. Beware that a bare,
30 unsigned number in that argument position, such as ""use String::Base
31 1"", will be interpreted as a version number to require of
32 "String::Base". It is therefore necessary to give the offset a leading
33 sign, or parenthesise it, or otherwise decorate it. The offset may be
34 any integer (positive, zero, or negative) within the range of Perl's
35 integer arithmetic.
36 A string index offset declaration is in effect from immediately after
38 the "use" line, until the end of the enclosing block or until
39 overridden by another string index offset declaration. A declared
40 offset always replaces the previous offset: they do not add. ""no
41 String::Base"" is equivalent to ""use String::Base +0"": it returns to
42 the Perlish state with zero offset.
43 A declared string index offset influences these types of operation:
45 • substring extraction ("substr($a, 3, 2)")
47 • substring splicing ("substr $a, 3, 2, "x"")
49 • substring searching ("index($a, "x")", "index($a, "x", 3)",
51 "rindex($a, "x")", "rindex($a, "x", 3)")
52 • string iterator position (pos($a))
54 Only forwards indexing, relative to the start of the string, is
56 supported. End-relative indexing, normally done using negative index
57 values, is not supported when an index offset is in effect. Use of an
58 index that is numerically less than the index offset will have
59 unpredictable results.
60 Differences from $[
62 This module is a replacement for the historical $[ variable. In early
63 Perl that variable was a runtime global, affecting all array and string
64 indexing in the program. In Perl 5, assignment to $[ acts as a
65 lexically-scoped pragma. $[ is deprecated. The original $[ was
66 removed in Perl 5.15.3, and later replaced in Perl 5.15.5 by an
67 automatically-loaded arybase module. This module reimplements the
68 index offset feature without any specific support from the core.
69 Unlike $[, this module does not affect indexing into arrays. This
71 module is concerned only with strings. To influence array indexing,
72 see Array::Base.
73 This module does not show the offset value in $[ or any other
75 accessible variable. With the string offset being lexically scoped,
76 there should be no need to write code to handle a variable offset.
77 $[ has some predictable, but somewhat strange, behaviour for indexes
79 less than the offset. The behaviour differs between substring
80 extraction and iterator positioning. This module does not attempt to
81 replicate it, and does not support end-relative indexing at all.
82 The string iterator position operator (pos($a)), as implemented by the
84 Perl core, generates a magical scalar which is linked to the underlying
85 string. The numerical value of the scalar varies if the iterator
86 position of the string is changed, and code with different $[ settings
87 will see accordingly different values. The scalar can also be written
88 to, to change the position of the string's iterator, and again the
89 interpretation of the value written varies according to the $[ setting
90 of the code that is doing the writing. This module does not replicate
91 any of that behaviour. With a string index offset from this module in
92 effect, pos($a) evaluates to an ordinary rvalue scalar, giving the
93 position of the string's iterator as it was at the time the operator
94 was evaluated, according to the string index offset in effect where the
95 operator appears.
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98 These methods are meant to be invoked on the "String::Base" package.
99 String::Base->import(BASE)
101 Sets up a string index offset of BASE, in the lexical environment
102 that is currently compiling.
103 String::Base->unimport
105 Clears the string index offset, in the lexical environment that is
106 currently compiling.
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109 B::Deparse will generate incorrect source when deparsing code that uses
110 a string index offset. It will include both the pragma to set up the
111 offset and the munged form of the affected operators. Either the
112 pragma or the munging is required to get the index offset effect; using
113 both will double the offset. Also, the code generated for a string
114 iterator position (pos($a)) operation involves a custom operator, which
115 B::Deparse can't understand, so the source it emits in that case is
116 completely wrong.
117 The additional operators generated by this module cause spurious
119 warnings if some of the affected string operations are used in void
120 context.
121 Prior to Perl 5.9.3, the lexical state of string index offset does not
123 propagate into string eval.
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126 Array::Base, arybase, "$[" in perlvar
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129 Andrew Main (Zefram) <zefram@fysh.org>
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132 Copyright (C) 2011, 2012, 2017 Andrew Main (Zefram) <zefram@fysh.org>
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135 This module is free software; you can redistribute it and/or modify it
136 under the same terms as Perl itself.
137perl v5.38.0 2023-07-21 String::Base(3pm)