To view the site and get a better experience from many other websitessimply upgrade to Internet Explorer or download an alternative browser such as FirefoxSafarior Google Chrome All of these browsers are free. You appear to have JavaScript disabled, or are running a non-JavaScript capable web browser To get the best experience, please enable JavaScript or download a modern web browser such as Internet Explorer 8FirefoxSafarior Google Chrome. In Perl, the operator determines what operation is performed, independent of the type of the operands. This is in contrast to many other dynamic languages, where the operation is determined by the type of the first argument. It also means that Perl has two versions of some operators, one for numeric and one for string comparison. Operator precedence means some operators are evaluated before others. This makes learning Perl easier for C folks. 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. These are all documented in perlfunc. If any list operator printetc. In the absence of parentheses, the precedence of list operators such as printsortor chmod is either very high or very low depending on whether you are looking at the left side or the right side of the operator. In other words, list operators tend to binary up all arguments that follow, and then act like a simple TERM with regard to the preceding expression. See perlreftut and perlref. Otherwise, the right side is a method name or a simple scalar variable containing either the method name or a subroutine reference, and the left side must be either an object a blessed reference or a class name that is, a package name. The dereferencing cases as opposed to method-calling cases are somewhat extended by the postderef feature. You just know it will be done sometime before or after the value is returned. This also means that modifying a variable twice in the same statement will lead to undefined behavior. The auto-increment operator has a little extra builtin magic to it. If you increment a variable that is numeric, or that has ever been used in a numeric context, you get a normal increment. See also not for a lower precedence version of this. Unary "-" performs arithmetic negation if the operand is numeric, including any string that looks like a number. If the operand is an identifier, a string consisting of a minus sign concatenated with the identifier is returned. Otherwise, if the string starts with a plus or minus, a string starting with the opposite sign is returned. When complementing strings, if all characters have ordinal values under 256, then their complements will, also. But if they do not, all characters will be in either or 64-bit complements, depending on your architecture. It is useful syntactically for separating a function name from a parenthesized expression that would otherwise be interpreted as the complete list of function arguments. Do not confuse this behavior with the behavior of backslash within a string, although both forms do convey the notion of protecting the next thing from interpolation. This operator makes that kind of operation work on some other string. The right argument is a search pattern, substitution, or transliteration. Behavior in list context depends on the particular operator. See Regexp Quote-Like Operators for details and perlretut for examples using these operators. If the right argument is an expression rather than a search pattern, substitution, or transliteration, it is interpreted as a search pattern at run time. This operator is not as well defined for negative operands, but it will execute faster. Binary "x" is the repetition operator. In scalar context or if the left operand is not enclosed in parentheses, it returns a string consisting of the left operand repeated the number of times specified by the right operand. Binary "-" returns the difference of two numbers. Arguments should be integers. See also Integer Arithmetic. If use integer see Integer Arithmetic is in force then signed C integers are used arithmetic shiftotherwise unsigned C integers login used logical shifteven for negative shiftees. In arithmetic right shift the sign bit is replicated on the left, in logical shift zero bits come in from the left. Shifting by negative number of bits means the reverse shift: left shift becomes right shift, right shift becomes left shift. This is unlike in C, where negative shift is undefined. Shifting by more bits than the size of the integers means most of the time zero all bits fall offexcept that under use integer right overshifting a negative shiftee results in This is unlike in C, where shifting by too many bits is undefined. For example, the relational operators in this section and the equality operators in the next one return for true and a special version of the defined empty string, ""which counts as a zero but is exempt from warnings about improper numeric conversions, just as "0 but true" is. Binary "lt" returns true if the left argument is stringwise less than the right argument. Binary "gt" returns true if the left argument is stringwise greater than the right argument. Binary "le" returns true if the left argument is stringwise less than or equal to the right argument. Binary "ge" returns true if the left argument is stringwise greater than or equal to the right argument. Binary "ne" returns true if the left argument is stringwise not equal to the right argument. Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise less than, equal to, or greater than the right argument. Smart matching is described in the next section. Do not mix these with Unicode, only use them with legacy 8-bit locale encodings. The standard Unicode::Collate and Unicode::Collate::Locale modules offer much more powerful solutions to collation issues. This is mostly used implicitly in the when construct described in perlsynalthough not all when clauses call the smartmatch operator. The smartmatch operator is experimental and its behavior is subject to change. It is also unique in that all other Perl operators impose a context usually string or numeric context on their operands, autoconverting those operands to those imposed contexts. In contrast, smartmatch infers contexts from the actual types of its operands and uses that type information to select a suitable comparison mechanism. It is often best read aloud as "in", "inside of", or "is contained in", because the left operand is often looked for inside the right operand. That makes the order of the operands to the smartmatch operand often opposite that of the regular match operator. In other words, the "smaller" thing is usually placed in the left operand and the larger one in the right. The behavior of a smartmatch depends on what type of things its arguments are, as determined by the following table. The first row of the table whose types apply determines the smartmatch behavior. For blessed references, the Object entries apply. Smartmatches involving hashes only consider hash keys, never hash values. The "like" code entry is not always an exact rendition. For example, the smartmatch operator short-circuits whenever possible, but grep does not. Smartmatching one hash against another reports whether both contain the same keys, no more and no less. This could be used to see whether two records have the same field names, without caring what values those fields might have. The smartmatch operator is most often used as the implicit operator of a when clause. See the section on "Switch Statements" in perlsyn. This is allowed to extend the usual smartmatch semantics. Using an object as the left operand is allowed, although not very useful. Smartmatching rules take precedence over overloading, so even if the object in the left operand has smartmatch overloading, this will be ignored. A left operand that is a non-overloaded object falls back on a string or numeric comparison of whatever the ref operator returns. Instead the above table is consulted as normal, and based on the type of Xoverloading may or may not be invoked. That is, if the left operand is false, the right operand is not even evaluated. Scalar or list context propagates down to the right operand if it is evaluated. Binary " " performs a short-circuit logical OR operation. That is, if the left operand is true, the right operand is not even evaluated. Usually, this is the same result as defined EXPR? EXPR EXPR except that the ternary-operator form can be used as a lvalue, while EXPR EXPR cannot. This is very useful for providing default values for variables. The short-circuit behavior is identical. In list context, it returns a list of values counting up by ones from the left value to the right value. If the left value is greater than the right value then it returns the empty list. The range operator is useful for writing foreach loops and for doing slice operations on arrays. In scalar context, ". The operator is bistable, like a flip-flop, and emulates the line-range comma operator of sedawkand various editors. It is false as long as its left operand is false. Once the left operand is true, the range operator stays true until the right operand is true, AFTER which the range operator becomes false again. It can test the right operand and become false on the same evaluation it became true as in awkbut it still returns true once. In all other regards, ". The right operand is not evaluated while the operator is in the "false" state, and the left operand is not evaluated while the operator is in the "true" state. The value returned is either the empty string for false, or a sequence number beginning with 1 for true. The sequence number is reset for each range encountered. You can exclude the beginning point by waiting for the sequence number to be greater than 1. If either operand of scalar ". If the final value specified is not in the sequence that the magical increment would produce, the sequence goes until the next value would be longer than the final value specified. It works much like an if-then-else. If the argument before the? These combined assignment operators can only operate on scalars, whereas the ordinary assignment operator can assign to arrays, hashes, lists and even references. See Context and List value constructors in perldataand Assigning to References in perlref. Unlike in C, the scalar assignment operator produces a valid lvalue. Modifying an assignment is equivalent to doing the assignment and then modifying the variable that was assigned to. See Bitwise String Operators. Binary "," is the comma operator. In scalar context it evaluates its left argument, throws that value away, then evaluates its right argument and returns that value. These arguments are also evaluated from left to right. This includes operands that might otherwise be interpreted as operators, constants, single number v-strings or function calls. If in doubt about this behavior, the left operand can be quoted explicitly. On the right side of a list binary, the comma has very low precedence, such that it controls all comma-separated expressions found there. Unary "not" returns the logical negation of the expression to its right. Binary "and" returns the logical conjunction of the two surrounding expressions. This means that it short-circuits: the right expression is evaluated only if the left expression is true. Binary "or" returns the logical disjunction of the two surrounding expressions. Due to its precedence, you must be careful to avoid using it as replacement for the operator. Binary "xor" returns the exclusive-OR of the two surrounding expressions. It cannot short-circuit of course. Perl provides customary quote characters for these behaviors, but also provides a way for you to choose your quote character for any of them. The Text::Balanced module standard as of v5. There can be whitespace between the operator and the quoting characters, except when is being used as the quoting character. Its argument will be taken from the next line. Only hexadecimal digits are valid between the braces. If an invalid character is encountered, a warning will be issued and the invalid character and all subsequent characters valid or invalid within the braces will be discarded. The result is the character specified by the hexadecimal number in the range 0x00 to 0xFF. Note that although the warning says the illegal character is ignored, it is only ignored as part of the escape and will still be used as the subsequent character in the string. For example Original Result Warns? On ASCII platforms, the resulting characters from the list above are the complete set of ASCII controls. Use of any other character following the "c" besides those listed above is discouraged, and as of Perl v5. Using the non-allowed characters generates a fatal error. It is a fatal error if there are no octal digits at all. The result is the character specified by the three-digit octal number in the range to but best to not use above 077, see next paragraph. Some contexts allow 2 or even 1 digit, but any usage without exactly three digits, the first being a zero, may give unintended results. Otherwise, it is best to use this construct only for ordinals and below, remembering to pad to the left with zeros to make three digits. Several constructs above specify a character by a number. If the number is x100, or above, Perl interprets it as a Unicode code point and the result is the corresponding Unicode character. That means that case-mapping a single character can sometimes produce a sequence of several characters. There is no such thing as an unvarying, physical newline character. It is only an illusion that the operating system, device drivers, C libraries, and Perl all conspire to preserve. Patterns are subject to an additional level of interpretation as a regular expression. This is done as a second pass, after variables are interpolated, so that regular expressions may be incorporated into the pattern from the variables. Apart from the behavior described above, Perl does not expand multiple levels of interpolation. In particular, contrary to the expectations of shell programmers, back-quotes do NOT interpolate within double quotes, nor do single quotes impede evaluation of variables when used within double quotes. This operator quotes and possibly compiles its STRING as a regular expression. The returned value is a normalized version of the original pattern. See perlre for additional information on valid syntax for STRINGand for a detailed look at the semantics of regular expressions. Searches a string for a pattern match, and in scalar context returns true if it succeeds, false if it fails. Options are as described in qr above; in addition, the following match process modifiers are available g Match globallyi. With the m you can use any pair of non-whitespace ASCII characters as delimiters. When using a delimiter character valid in an identifier, whitespace is required after the m PATTERN may contain variables, which will be interpolated every time the pattern search is evaluated, except for when the delimiter is a single quote. Perl will not recompile the pattern unless an interpolated variable that it contains changes. Once upon a time, Perl binary recompile regular expressions unnecessarily, and this modifier was useful to tell it not to do so, in the interests of speed. If the PATTERN evaluates to the empty string, the last successfully matched regular expression is used instead. In this case, only the g and c flags on the empty pattern are honored; the other flags are taken from the original pattern. If no match has previously succeeded, this will silently act instead as a genuine empty pattern which will always match. In all of these examples, Perl will assume you meant defined-or. When there are no parentheses in the pattern, the return value is the list for success. How it behaves depends on the context. In list context, it returns a list of the substrings matched by any capturing parentheses in the regular expression. If there are no parentheses, it returns a list of all the matched strings, as if there were parentheses around the whole pattern. The position after the last match can be read or set using the pos function; see pos. A useful idiom for lex -like scanners is G. This is a useful optimization when you want to see only the first occurrence of something in each file of a set of files, for instance. In the past, the leading m in m? As of vomitting it produces a syntax error. If you encounter this construct in older code, you can just add m. Searches a string for a pattern, and if found, replaces that pattern with the replacement text and returns the number of substitutions made. Otherwise it returns false specifically, the empty string. The copy will always be a plain string, even if the input is an object or a tied variable. If the delimiter chosen is a single quote, no interpolation is done on either the PATTERN or the REPLACEMENT. If the pattern evaluates to the empty string, the last successfully executed regular expression is used instead. See perlre for further explanation on these. Add space after the s when using a character allowed in identifiers. Note that Perl treats backticks as normal delimiters; the replacement text is not evaluated as a command. It is, however, syntax checked at compile-time. Shell wildcards, pipes, and redirections will be honored. The collected standard output of the command is returned; standard error is unaffected. In scalar context, it comes back as a single potentially multi-line string, or undef if the command failed. Because backticks do not affect standard error, use shell file descriptor syntax assuming the shell supports this if you care to address this. On most platforms, you will have to protect shell metacharacters if you want them treated literally. See perlsec for a clean and safe example of a manual fork and exec to emulate backticks safely. On some platforms notably DOS-like onesthe shell may not be capable of dealing with multiline commands, so putting newlines in the string may not get you what you want. Beware that some command shells may place restrictions on the length of the command line. See the platform-specific release notes for more details about your particular environment. Using this operator can lead to programs that are difficult to port, because the shell commands called vary between systems, and may in fact not be present at all. As one example, the type command under the POSIX shell is very different from the type command under DOS. Perl was made to be a glue language, and one of the things it glues together is commands. Evaluates to a list of the words extracted out of STRINGusing embedded whitespace as the word delimiters. Transliterates all occurrences of the characters found in the search list with the corresponding character in the replacement list. It returns the number of characters replaced or deleted. The new copy is always a plain string, even if the input string is an object or a tied variable. A hyphen at the beginning or end, or preceded by a backslash is considered a literal. Escape sequence details are in the table near the beginning of this section. Most ranges are unportable between character sets, but certain ones signal Perl to do special handling to make them portable. There are two classes of portable ranges. This is a portable range, and has the same effect on every platform it is run on. It turns out that in this example, these are the ASCII printable characters. But, even for portable ranges, it is not generally obvious what is included without having to look things up. Note that this is slightly more flexible than the behavior of some tr programs, which delete anything they find in the SEARCHLISTperiod. Otherwise, if the REPLACEMENTLIST is shorter than the SEARCHLISTthe final character is replicated till it is long enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. Because the transliteration table is built at compile time, neither the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote interpolation. The terminating string may be either an identifier a wordor some quoted text. An unquoted identifier works like double quotes. If you put a space it will be treated as a null identifier, which is valid, and matches the first empty line. The terminating string must appear by itself unquoted and with no surrounding whitespace on the terminating line. This is similar to single quoted strings except that backslashes have no special meaning, with being treated as two backslashes and not one as they would in every other quoting construct. The content of the here doc is treated just as it would be if the string were embedded in backticks. The general rule is that the identifier must be a string literal. Stick with that, and you should be safe. This strategy is so successful that Perl programmers often do not suspect the ambivalence of what they write. This section hopes to clarify how Perl handles quoted constructs. Although the most common reason to learn this is to unravel labyrinthine regular expressions, because the initial steps of parsing are the same for all quoting operators, they are all discussed together. The most important Perl parsing rule is the first one discussed below: when login a quoted construct, Perl first finds the end of that construct, then interprets its contents. If you understand this rule, you may skip the rest of this section on the first reading. Some passes discussed below are performed concurrently, but because their results are the same, we consider them individually. For different quoting constructs, Perl performs different numbers of passes, from one to four, but these passes are always performed in the same order. The first pass is finding the end of the quoted construct. This results in saving to a safe location a copy of the text between the starting and ending delimitersnormalized as necessary to avoid needing to know what the original delimiters were. If the construct is a here-doc, the ending delimiter is a line that has a terminating string as the content. When searching for the terminating line of a here-doc, nothing is skipped. In other words, lines after the here-doc syntax are compared with the terminating string line by line. For the constructs except here-docs, single characters are used as starting and ending delimiters. If the starting delimiter is an unpaired character like or a closing punctuation, the ending delimiter is the same as the starting delimiter. When searching for single-character delimiters, escaped delimiters and are skipped. If the delimiters are bracketing, nested pairs are also skipped. However, when backslashes are used as the delimiters like qq and trnothing is skipped. During the search for the end, backslashes that escape delimiters or other backslashes are removed exactly speaking, they are not copied to the safe location. For constructs with three-part delimiters syand trthe search is repeated once more. If login first delimiter is not an opening punctuation, the three delimiters must be the same, such as s!!! The next step is interpolation in the text obtained, which is now delimiter-independent. There are multiple cases. No interpolation is performed. Note that the combination is left intact, since escaped delimiters are not available for here-docs. No interpolation is performed at this stage. Any backslashed sequences including are treated at the stage to parsing regular expressions. Note also that the interpolation code needs to make a decision on where the interpolated scalar ends. However any other combinations of followed by a character are not substituted but only skipped, in order to parse them as regular expressions at the following step. The lack of processing of creates specific restrictions on the post-processed text. This step is the last one for all constructs except regular expressions, which are processed further. Previous steps were performed during the compilation of Perl code, but this one happens at run time, although it may be optimized to be calculated at compile time if appropriate. After preprocessing described above, and possibly after evaluation if concatenation, joining, casing translation, or metaquoting are involved, the resulting string is passed to the RE engine for compilation. Whatever happens in the RE engine might be better discussed in perlrebut for the sake of continuity, we shall do so here. The RE engine scans the string from left to right and converts it into a finite automaton. The terminator of runtime? It is possible to inspect both the string given to RE engine and the resulting finite automaton. 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 without notice. This step is performed over the finite automaton that was generated during the previous pass. It is then interpreted as an external command, and the output of that command is the value of the backtick string, like in a shell. In scalar context, a single string consisting of all output is returned. In list context, a list of values is returned, one per line of output. The command is executed each time the pseudo-literal is evaluated. Unlike in cshno translation is done on the return data--newlines remain newlines. Unlike in any of the shells, single quotes do not hide variable names in the command from interpretation. To pass a literal dollar-sign through to the shell you need to hide it with a backslash. The generalized form of backticks is qx Because backticks always undergo shell expansion as well, see perlsec for security concerns. In scalar context, evaluating a filehandle in angle brackets yields the next line from that file the newline, if any, includedor undef at end-of-file or on error. Ordinarily you must assign the returned value to a variable, but there is one situation where an automatic assignment happens. The defined test avoids problems where the line has a string value that would be treated as false by Perl; for example a "" or a "0" with no trailing newline. The filehandles STDIN, STDOUT, and STDERR are predefined. The filehandles stdinstdoutand 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, amongst others. See perlopentut and open for details on this. The ARGV array is then processed as a list of filenames. It also uses filehandle ARGV internally. See the example in eof for how to reset line numbers on each file. If you want to set ARGV to your own list of files, go right ahead. This distinction is determined on syntactic grounds alone. All values must be read before it will start over. As with filehandle reads, an automatic defined is generated when the glob occurs in the test part of a whilebecause legal glob returns for example, a file called would otherwise terminate the loop. Again, undef is returned only once. Backslash interpolation also happens at compile time. The behavior of these operators is problematic and subject to change if either or both of the strings are encoded in UTF-8 see Byte and Character Semantics in perlunicode. See vec for information on how to manipulate individual bits in a bit vector use integer ; you may tell the compiler to use integer operations see integer for a detailed explanation from here to the end of the enclosing BLOCK. An inner BLOCK may countermand this by saying no integer ; which lasts until the end of that BLOCK. But see also Bitwise String Operators However, use integer still has meaning for them. By default, their results are interpreted as unsigned integers, but if use integer is in effect, their results are interpreted as signed integers. While use integer provides integer-only arithmetic, there is no analogous mechanism to provide automatic rounding or truncation to a certain number of decimal places. For rounding to a certain number of digits, sprintf or printf is usually the easiest route. Floating-point numbers are only approximations to what a mathematician would call real numbers. There are infinitely more reals than floats, so some corners must be cut. Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system rounding is being used by Perl, but to instead implement the rounding function you need yourself.
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