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Diffstat (limited to 'contrib/perl5/pod/perlop.pod')
| -rw-r--r-- | contrib/perl5/pod/perlop.pod | 276 |
1 files changed, 193 insertions, 83 deletions
diff --git a/contrib/perl5/pod/perlop.pod b/contrib/perl5/pod/perlop.pod index c7209fa..9f6d965 100644 --- a/contrib/perl5/pod/perlop.pod +++ b/contrib/perl5/pod/perlop.pod @@ -44,7 +44,7 @@ Many operators can be overloaded for objects. See L<overload>. =head2 Terms and List Operators (Leftward) -A TERM has the highest precedence in Perl. They includes variables, +A TERM has the highest precedence in Perl. They include variables, quote and quote-like operators, any expression in parentheses, and any function whose arguments are parenthesized. Actually, there aren't really functions in this sense, just list operators and unary @@ -620,9 +620,9 @@ the same character fore and aft, but the 4 sorts of brackets "" qq{} Literal yes `` qx{} Command yes (unless '' is delimiter) qw{} Word list no - // m{} Pattern match yes - qr{} Pattern yes - s{}{} Substitution yes + // m{} Pattern match yes (unless '' is delimiter) + qr{} Pattern yes (unless '' is delimiter) + s{}{} Substitution yes (unless '' is delimiter) tr{}{} Transliteration no (but see below) Note that there can be whitespace between the operator and the quoting @@ -645,8 +645,8 @@ a transliteration, the first ten of these sequences may be used. \b backspace (BS) \a alarm (bell) (BEL) \e escape (ESC) - \033 octal char - \x1b hex char + \033 octal char (ESC) + \x1b hex char (ESC) \c[ control char \l lowercase next char @@ -752,22 +752,22 @@ Options are: If "/" is the delimiter then the initial C<m> is optional. With the C<m> you can use any pair of non-alphanumeric, non-whitespace characters -as delimiters (if single quotes are used, no interpretation is done -on the replacement string. Unlike Perl 4, Perl 5 treats backticks as normal -delimiters; the replacement text is not evaluated as a command). -This is particularly useful for matching Unix path names -that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is +as delimiters. This is particularly useful for matching Unix path names +that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is the delimiter, then the match-only-once rule of C<?PATTERN?> applies. +If "'" is the delimiter, no variable interpolation is performed on the +PATTERN. PATTERN may contain variables, which will be interpolated (and the -pattern recompiled) every time the pattern search is evaluated. (Note -that C<$)> and C<$|> might not be interpolated because they look like -end-of-string tests.) If you want such a pattern to be compiled only -once, add a C</o> after the trailing delimiter. This avoids expensive -run-time recompilations, and is useful when the value you are -interpolating won't change over the life of the script. However, mentioning -C</o> constitutes a promise that you won't change the variables in the pattern. -If you change them, Perl won't even notice. +pattern recompiled) every time the pattern search is evaluated, except +for when the delimiter is a single quote. (Note that C<$)> and C<$|> +might not be interpolated because they look like end-of-string tests.) +If you want such a pattern to be compiled only once, add a C</o> after +the trailing delimiter. This avoids expensive run-time recompilations, +and is useful when the value you are interpolating won't change over +the life of the script. However, mentioning C</o> constitutes a promise +that you won't change the variables in the pattern. If you change them, +Perl won't even notice. If the PATTERN evaluates to the empty string, the last I<successfully> matched regular expression is used instead. @@ -829,10 +829,12 @@ Examples: ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); # scalar context - $/ = ""; $* = 1; # $* deprecated in modern perls - while (defined($paragraph = <>)) { - while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { - $sentences++; + { + local $/ = ""; + while (defined($paragraph = <>)) { + while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { + $sentences++; + } } } print "$sentences\n"; @@ -907,14 +909,50 @@ A double-quoted, interpolated string. if /(tcl|rexx|python)/; # :-) $baz = "\n"; # a one-character string -=item qr/STRING/imosx +=item qr/PATTERN/imosx + +Quote-as-a-regular-expression operator. I<STRING> is interpolated the +same way as I<PATTERN> in C<m/PATTERN/>. If "'" is used as the +delimiter, no variable interpolation is done. Returns a Perl value +which may be used instead of the corresponding C</STRING/imosx> expression. + +For example, + + $rex = qr/my.STRING/is; + s/$rex/foo/; -A string which is (possibly) interpolated and then compiled as a -regular expression. The result may be used as a pattern in a match +is equivalent to + + s/my.STRING/foo/is; + +The result may be used as a subpattern in a match: $re = qr/$pattern/; $string =~ /foo${re}bar/; # can be interpolated in other patterns $string =~ $re; # or used standalone + $string =~ /$re/; # or this way + +Since Perl may compile the pattern at the moment of execution of qr() +operator, using qr() may have speed advantages in I<some> situations, +notably if the result of qr() is used standalone: + + sub match { + my $patterns = shift; + my @compiled = map qr/$_/i, @$patterns; + grep { + my $success = 0; + foreach my $pat @compiled { + $success = 1, last if /$pat/; + } + $success; + } @_; + } + +Precompilation of the pattern into an internal representation at the +moment of qr() avoids a need to recompile the pattern every time a +match C</$pat/> is attempted. (Note that Perl has many other +internal optimizations, but none would be triggered in the above +example if we did not use qr() operator.) Options are: @@ -924,19 +962,6 @@ Options are: s Treat string as single line. x Use extended regular expressions. -The benefit from this is that the pattern is precompiled into an internal -representation, and does not need to be recompiled every time a match -is attempted. This makes it very efficient to do something like: - - foreach $pattern (@pattern_list) { - my $re = qr/$pattern/; - foreach $line (@lines) { - if($line =~ /$re/) { - do_something($line); - } - } - } - See L<perlre> for additional information on valid syntax for STRING, and for a detailed look at the semantics of regular expressions. @@ -1023,6 +1048,12 @@ whitespace as the word delimiters. It is exactly equivalent to This equivalency means that if used in scalar context, you'll get split's (unfortunate) scalar context behavior, complete with mysterious warnings. +However do not rely on this as in a future release it could be changed to +be exactly equivalent to the list + + ('foo', 'bar', 'baz') + +Which in a scalar context would result in C<'baz'>. Some frequently seen examples: @@ -1045,7 +1076,7 @@ variable is searched and modified. (The string specified with C<=~> must be scalar variable, an array element, a hash element, or an assignment to one of those, i.e., an lvalue.) -If the delimiter chosen is single quote, no variable interpolation is +If the delimiter chosen is a single quote, no variable interpolation is done on either the PATTERN or the REPLACEMENT. Otherwise, if the PATTERN contains a $ that looks like a variable rather than an end-of-string test, the variable will be interpolated into the pattern @@ -1148,6 +1179,7 @@ the number of characters replaced or deleted. If no string is specified via the =~ or !~ operator, the $_ string is transliterated. (The string specified with =~ must be a scalar variable, an array element, a hash element, or an assignment to one of those, i.e., an lvalue.) + A character range may be specified with a hyphen, so C<tr/A-J/0-9/> does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>. For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the @@ -1155,6 +1187,13 @@ SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has its own pair of quotes, which may or may not be bracketing quotes, e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>. +Note also that the whole range idea is rather unportable between +character sets--and even within character sets they may cause results +you probably didn't expect. A sound principle is to use only ranges +that begin from and end at either alphabets of equal case (a-e, A-E), +or digits (0-4). Anything else is unsafe. If in doubt, spell out the +character sets in full. + Options: c Complement the SEARCHLIST. @@ -1229,6 +1268,13 @@ details discussed in this section is hairy regular expressions. However, the first steps of parsing are the same for all Perl quoting operators, so here they are discussed together. +The most important detail of Perl parsing rules is the first one +discussed below; when processing a quoted construct, Perl I<first> +finds the end of the construct, then it interprets the contents of the +construct. If you understand this rule, you may skip the rest of this +section on the first reading. The other rules would +contradict user's expectations much less frequently than the first one. + Some of the passes discussed below are performed concurrently, but as far as results are the same, we consider them one-by-one. For different quoting constructs Perl performs different number of passes, from @@ -1238,32 +1284,37 @@ one to five, but they are always performed in the same order. =item Finding the end -First pass is finding the end of the quoted construct, be it multichar ender +First pass is finding the end of the quoted construct, be it +a multichar delimiter C<"\nEOF\n"> of C<<<EOF> construct, C</> which terminates C<qq/> construct, C<]> which terminates C<qq[> construct, or C<E<gt>> which terminates a fileglob started with C<<>. -When searching for multichar construct no skipping is performed. When -searching for one-char non-matching delimiter, such as C</>, combinations +When searching for one-char non-matching delimiter, such as C</>, combinations C<\\> and C<\/> are skipped. When searching for one-char matching delimiter, such as C<]>, combinations C<\\>, C<\]> and C<\[> are skipped, and -nested C<[>, C<]> are skipped as well. +nested C<[>, C<]> are skipped as well. When searching for multichar delimiter +no skipping is performed. -For 3-parts constructs, C<s///> etc. the search is repeated once more. +For constructs with 3-part delimiters (C<s///> etc.) the search is +repeated once more. -During this search no attention is paid to the semantic of the construct, thus +During this search no attention is paid to the semantic of the construct, +thus: "$hash{"$foo/$bar"}" -or +or: m/ - bar # This is not a comment, this slash / terminated m//! + bar # NOT a comment, this slash / terminated m//! /x -do not form legal quoted expressions. Note that since the slash which -terminated C<m//> was followed by a C<SPACE>, this is not C<m//x>, -thus C<#> was interpreted as a literal C<#>. +do not form legal quoted expressions, the quoted part ends on the first C<"> +and C</>, and the rest happens to be a syntax error. Note that since the slash +which terminated C<m//> was followed by a C<SPACE>, the above is not C<m//x>, +but rather C<m//> with no 'x' switch. So the embedded C<#> is interpreted +as a literal C<#>. =item Removal of backslashes before delimiters @@ -1297,42 +1348,64 @@ The only interpolation is removal of C<\> from pairs C<\\>. =item C<"">, C<``>, C<qq//>, C<qx//>, C<<file*globE<gt>> C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are converted -to corresponding Perl constructs, thus C<"$foo\Qbaz$bar"> is converted to +to corresponding Perl constructs, thus C<"$foo\Qbaz$bar"> is converted to : $foo . (quotemeta("baz" . $bar)); Other combinations of C<\> with following chars are substituted with -appropriate expansions. +appropriate expansions. + +Let it be stressed that I<whatever is between C<\Q> and C<\E>> is interpolated +in the usual way. Say, C<"\Q\\E"> has no C<\E> inside: it has C<\Q>, C<\\>, +and C<E>, thus the result is the same as for C<"\\\\E">. Generally speaking, +having backslashes between C<\Q> and C<\E> may lead to counterintuitive +results. So, C<"\Q\t\E"> is converted to: + + quotemeta("\t") + +which is the same as C<"\\\t"> (since TAB is not alphanumerical). Note also +that: -Interpolated scalars and arrays are converted to C<join> and C<.> Perl -constructs, thus C<"'@arr'"> becomes + $str = '\t'; + return "\Q$str"; - "'" . (join $", @arr) . "'"; +may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">. -Since all three above steps are performed simultaneously left-to-right, -the is no way to insert a literal C<$> or C<@> inside C<\Q\E> pair: it -cannot be protected by C<\>, since any C<\> (except in C<\E>) is -interpreted as a literal inside C<\Q\E>, and any C<$> is +Interpolated scalars and arrays are internally converted to the C<join> and +C<.> Perl operations, thus C<"$foo >>> '@arr'"> becomes: + + $foo . " >>> '" . (join $", @arr) . "'"; + +All the operations in the above are performed simultaneously left-to-right. + +Since the result of "\Q STRING \E" has all the metacharacters quoted +there is no way to insert a literal C<$> or C<@> inside a C<\Q\E> pair: if +protected by C<\> C<$> will be quoted to became "\\\$", if not, it is interpreted as starting an interpolated scalar. -Note also that the interpolating code needs to make decision where the -interpolated scalar ends, say, whether C<"a $b -E<gt> {c}"> means +Note also that the interpolating code needs to make a decision on where the +interpolated scalar ends. For instance, whether C<"a $b -E<gt> {c}"> means: "a " . $b . " -> {c}"; -or +or: "a " . $b -> {c}; -Most the time the decision is to take the longest possible text which does -not include spaces between components and contains matching braces/brackets. +I<Most of the time> the decision is to take the longest possible text which +does not include spaces between components and contains matching +braces/brackets. Since the outcome may be determined by I<voting> based +on heuristic estimators, the result I<is not strictly predictable>, but +is usually correct for the ambiguous cases. =item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>, Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> and interpolation happens (almost) as with C<qq//> constructs, but I<the substitution of C<\> followed by -other chars is not performed>! Moreover, inside C<(?{BLOCK})> no processing -is performed at all. +RE-special chars (including C<\>) is not performed>! Moreover, +inside C<(?{BLOCK})>, C<(?# comment )>, and C<#>-comment of +C<//x>-regular expressions no processing is performed at all. +This is the first step where presence of the C<//x> switch is relevant. Interpolation has several quirks: C<$|>, C<$(> and C<$)> are not interpolated, and constructs C<$var[SOMETHING]> are I<voted> (by several different estimators) @@ -1340,15 +1413,25 @@ to be an array element or C<$var> followed by a RE alternative. This is the place where the notation C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as an array element C<-9>, not as a regular expression from variable C<$arr> followed by a digit, which is the interpretation of -C</$arr[0-9]/>. +C</$arr[0-9]/>. Since voting among different estimators may be performed, +the result I<is not predictable>. + +It is on this step that C<\1> is converted to C<$1> in the replacement +text of C<s///>. Note that absence of processing of C<\\> creates specific restrictions on the post-processed text: if the delimiter is C</>, one cannot get the combination C<\/> into the result of this step: C</> will finish the regular expression, C<\/> will be stripped to C</> on the previous step, and C<\\/> will be left as is. Since C</> is equivalent to C<\/> inside a regular expression, this -does not matter unless the delimiter is special character for the RE engine, as -in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>. +does not matter unless the delimiter is a special character for the RE engine, +as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>, or an alphanumeric char, as in: + + m m ^ a \s* b mmx; + +In the above RE, which is intentionally obfuscated for illustration, the +delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the +RE is the same as for C<m/ ^ a s* b /mx>). =back @@ -1367,32 +1450,48 @@ engine for compilation. Whatever happens in the RE engine is better be discussed in L<perlre>, but for the sake of continuity let us do it here. -This is the first step where presence of the C<//x> switch is relevant. +This is another step where presence of the C<//x> switch is relevant. The RE engine scans the string left-to-right, and converts it to a finite automaton. Backslashed chars are either substituted by corresponding literal -strings, or generate special nodes of the finite automaton. Characters -which are special to the RE engine generate corresponding nodes. C<(?#...)> +strings (as with C<\{>), or generate special nodes of the finite automaton +(as with C<\b>). Characters which are special to the RE engine (such as +C<|>) generate corresponding nodes or groups of nodes. C<(?#...)> comments are ignored. All the rest is either converted to literal strings to match, or is ignored (as is whitespace and C<#>-style comments if C<//x> is present). Note that the parsing of the construct C<[...]> is performed using -absolutely different rules than the rest of the regular expression. -Similarly, the C<(?{...})> is only checked for matching braces. +rather different rules than for the rest of the regular expression. +The terminator of this construct is found using the same rules as for +finding a terminator of a C<{}>-delimited construct, the only exception +being that C<]> immediately following C<[> is considered as if preceded +by a backslash. Similarly, the terminator of C<(?{...})> is found using +the same rules as for finding a terminator of a C<{}>-delimited construct. + +It is possible to inspect both the string given to RE engine, and the +resulting finite automaton. See arguments C<debug>/C<debugcolor> +of C<use L<re>> directive, and/or B<-Dr> option of Perl in +L<perlrun/Switches>. =item Optimization of regular expressions This step is listed for completeness only. Since it does not change semantics, details of this step are not documented and are subject -to change. +to change. This step is performed over the finite automaton generated +during the previous pass. + +However, in older versions of Perl C<L<split>> used to silently +optimize C</^/> to mean C</^/m>. This behaviour, though present +in current versions of Perl, may be deprecated in future. =back =head2 I/O Operators There are several I/O operators you should know about. + A string enclosed by backticks (grave accents) first undergoes variable substitution just like a double quoted string. It is then interpreted as a command, and the output of that command is the value @@ -1410,9 +1509,13 @@ The generalized form of backticks is C<qx//>. (Because backticks always undergo shell expansion as well, see L<perlsec> for security concerns.) -Evaluating a filehandle in angle brackets yields the next line from -that file (newline, if any, included), or C<undef> at end of file. -Ordinarily you must assign that value to a variable, but there is one +In a scalar context, evaluating a filehandle in angle brackets yields the +next line from that file (newline, if any, included), or C<undef> at +end-of-file. When C<$/> is set to C<undef> (i.e. file slurp mode), +and the file is empty, it returns C<''> the first time, followed by +C<undef> subsequently. + +Ordinarily you must assign the returned value to a variable, but there is one situation where an automatic assignment happens. I<If and ONLY if> the input symbol is the only thing inside the conditional of a C<while> or C<for(;;)> loop, the value is automatically assigned to the variable @@ -1449,13 +1552,16 @@ The filehandles STDIN, STDOUT, and STDERR are predefined. (The filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in packages, where they would be interpreted as local identifiers rather than global.) Additional filehandles may be created with the open() -function. See L<perlfunc/open()> for details on this. +function. See L<perlfunc/open> for details on this. If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for a list, a list consisting of all the input lines is returned, one line per list element. It's easy to make a I<LARGE> data space this way, so use with care. +E<lt>FILEHANDLEE<gt> may also be spelt readline(FILEHANDLE). See +L<perlfunc/readline>. + The null filehandle E<lt>E<gt> is special and can be used to emulate the behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from standard input, or from each file listed on the command line. Here's @@ -1622,9 +1728,10 @@ Bitstrings of any size may be manipulated by the bitwise operators (C<~ | & ^>). If the operands to a binary bitwise op are strings of different sizes, -B<or> and B<xor> ops will act as if the shorter operand had additional -zero bits on the right, while the B<and> op will act as if the longer -operand were truncated to the length of the shorter. +B<|> and B<^> ops will act as if the shorter operand had additional +zero bits on the right, while the B<&> op will act as if the longer +operand were truncated to the length of the shorter. Note that the +granularity for such extension or truncation is one or more I<bytes>. # ASCII-based examples print "j p \n" ^ " a h"; # prints "JAPH\n" @@ -1645,6 +1752,9 @@ operation you intend by using C<""> or C<0+>, as in the examples below. $baz = 0+$foo & 0+$bar; # both ops explicitly numeric $biz = "$foo" ^ "$bar"; # both ops explicitly stringy +See L<perlfunc/vec> for information on how to manipulate individual bits +in a bit vector. + =head2 Integer Arithmetic By default Perl assumes that it must do most of its arithmetic in |
