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 ```PERLOP(1) Perl Programmers Reference Guide PERLOP(1) NAME perlop - Perl operators and precedence DESCRIPTION Operator Precedence and Associativity Operator precedence and associativity work in Perl more or less like they do in mathematics. Operator precedence means some operators are evaluated before others. For example, in "2 + 4 * 5", the multiplication has higher precedence so "4 * 5" is evaluated first yielding "2 + 20 == 22" and not "6 * 5 == 30". Operator associativity defines what happens if a sequence of the same operators is used one after another: whether the evaluator will evalu ate the left operations first or the right. For example, in "8 - 4 - 2", subtraction is left associative so Perl evaluates the expression left to right. "8 - 4" is evaluated first making the expression "4 - 2 == 2" and not "8 - 2 == 6". Perl operators have the following associativity and precedence, listed from highest precedence to lowest. Operators borrowed from C keep the same precedence relationship with each other, even where Cs precedence is slightly screwy. (This makes learning Perl easier for C folks.) With very few exceptions, these all operate on scalar values only, not array values. left terms and list operators (leftward) left -> nonassoc ++ -- right ** right ! ~ \ and unary + and - left =~ !~ left * / % x left + - . left << >> nonassoc named unary operators nonassoc < > <= >= lt gt le ge nonassoc == != <=> eq ne cmp left & left | ^ left && left || nonassoc .. ... right ?: right = += -= *= etc. left , => nonassoc list operators (rightward) right not left and left or xor In the following sections, these operators are covered in precedence order. Many operators can be overloaded for objects. See overload. Terms and List Operators (Leftward) 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 arent really functions in this sense, just list operators and unary operators behaving as functions because you put parentheses around the arguments. These are all documented in perlfunc. If any list operator (print(), etc.) or any unary operator (chdir(), etc.) is followed by a left parenthesis as the next token, the opera tor and arguments within parentheses are taken to be of highest prece dence, just like a normal function call. In the absence of parentheses, the precedence of list operators such as "print", "sort", or "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. For example, in @ary = (1, 3, sort 4, 2); print @ary; # prints 1324 the commas on the right of the sort are evaluated before the sort, but the commas on the left are evaluated after. In other words, list oper ators tend to gobble up all arguments that follow, and then act like a simple TERM with regard to the preceding expression. Be careful with parentheses: # These evaluate exit before doing the print: print(\$foo, exit); # Obviously not what you want. print \$foo, exit; # Nor is this. # These do the print before evaluating exit: (print \$foo), exit; # This is what you want. print(\$foo), exit; # Or this. print (\$foo), exit; # Or even this. Also note that print (\$foo & 255) + 1, "\n"; probably doesnt do what you expect at first glance. The parentheses enclose the argument list for "print" which is evaluated (printing the result of "\$foo & 255"). Then one is added to the return value of "print" (usually 1). The result is something like this: 1 + 1, "\n"; # Obviously not what you meant. To do what you meant properly, you must write: print((\$foo & 255) + 1, "\n"); See "Named Unary Operators" for more discussion of this. Also parsed as terms are the "do {}" and "eval {}" constructs, as well as subroutine and method calls, and the anonymous constructors "[]" and "{}". See also "Quote and Quote-like Operators" toward the end of this sec tion, as well as "I/O Operators". The Arrow Operator ""->"" is an infix dereference operator, just as it is in C and C++. If the right side is either a "[...]", "{...}", or a "(...)" subscript, then the left side must be either a hard or symbolic reference to an array, a hash, or a subroutine respectively. (Or technically speaking, a location capable of holding a hard reference, if its an array or hash reference being used for assignment.) 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). See perlobj. Auto-increment and Auto-decrement "++" and "--" work as in C. That is, if placed before a variable, they increment or decrement the variable by one before returning the value, and if placed after, increment or decrement after returning the value. \$i = 0; \$j = 0; print \$i++; # prints 0 print ++\$j; # prints 1 Note that just as in C, Perl doesnt define when the variable is incre mented or decremented. 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 behaviour. Avoid statements like: \$i = \$i ++; print ++ \$i + \$i ++; Perl will not guarantee what the result of the above statements is. 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. If, however, the vari able has been used in only string contexts since it was set, and has a value that is not the empty string and matches the pattern "/^[a-zA-Z]*[0-9]*\z/", the increment is done as a string, preserving each character within its range, with carry: print ++(\$foo = 99); # prints 100 print ++(\$foo = a0); # prints a1 print ++(\$foo = Az); # prints Ba print ++(\$foo = zz); # prints aaa "undef" is always treated as numeric, and in particular is changed to 0 before incrementing (so that a post-increment of an undef value will return 0 rather than "undef"). The auto-decrement operator is not magical. Exponentiation Binary "**" is the exponentiation operator. It binds even more tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is imple mented using Cs pow(3) function, which actually works on doubles internally.) Symbolic Unary Operators Unary "!" performs logical negation, i.e., "not". See also "not" for a lower precedence version of this. Unary "-" performs arithmetic negation if the operand is numeric. If the operand is an identifier, a string consisting of a minus sign con catenated with the identifier is returned. Otherwise, if the string starts with a plus or minus, a string starting with the opposite sign is returned. One effect of these rules is that -bareword is equivalent to the string "-bareword". If, however, the string begins with a non- alphabetic character (exluding "+" or "-"), Perl will attempt to con vert the string to a numeric and the arithmetic negation is performed. If the string cannot be cleanly converted to a numeric, Perl will give the warning Argument "the string" isnt numeric in negation (-) at .... Unary "~" performs bitwise negation, i.e., 1s complement. For exam ple, "0666 & ~027" is 0640. (See also "Integer Arithmetic" and "Bit wise String Operators".) Note that the width of the result is plat form-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 bits wide on a 64-bit platform, so if you are expecting a certain bit width, remember to use the & operator to mask off the excess bits. Unary "+" has no effect whatsoever, even on strings. It is useful syn tactically for separating a function name from a parenthesized expres sion that would otherwise be interpreted as the complete list of func tion arguments. (See examples above under "Terms and List Operators (Leftward)".) Unary "\" creates a reference to whatever follows it. See perlreftut and perlref. Do not confuse this behavior with the behavior of back slash within a string, although both forms do convey the notion of pro tecting the next thing from interpolation. Binding Operators Binary "=~" binds a scalar expression to a pattern match. Certain operations search or modify the string \$_ by default. This operator makes that kind of operation work on some other string. The right argument is a search pattern, substitution, or transliteration. The left argument is what is supposed to be searched, substituted, or transliterated instead of the default \$_. When used in scalar context, the return value generally indicates the success of the operation. Behavior in list context depends on the particular operator. See "Reg exp 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. Binary "!~" is just like "=~" except the return value is negated in the logical sense. Multiplicative Operators Binary "*" multiplies two numbers. Binary "/" divides two numbers. Binary "%" computes the modulus of two numbers. Given integer operands \$a and \$b: If \$b is positive, then "\$a % \$b" is \$a minus the largest multiple of \$b that is not greater than \$a. If \$b is negative, then "\$a % \$b" is \$a minus the smallest multiple of \$b that is not less than \$a (i.e. the result will be less than or equal to zero). Note that when "use integer" is in scope, "%" gives you direct access to the mod ulus operator as implemented by your C compiler. 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 con sisting of the left operand repeated the number of times specified by the right operand. In list context, if the left operand is enclosed in parentheses or is a list formed by "qw/STRING/", it repeats the list. If the right operand is zero or negative, it returns an empty string or an empty list, depending on the context. print - x 80; # print row of dashes print "\t" x (\$tab/8), x (\$tab%8); # tab over @ones = (1) x 80; # a list of 80 1s @ones = (5) x @ones; # set all elements to 5 Additive Operators Binary "+" returns the sum of two numbers. Binary "-" returns the difference of two numbers. Binary "." concatenates two strings. Shift Operators Binary "<<" returns the value of its left argument shifted left by the number of bits specified by the right argument. Arguments should be integers. (See also "Integer Arithmetic".) Binary ">>" returns the value of its left argument shifted right by the number of bits specified by the right argument. Arguments should be integers. (See also "Integer Arithmetic".) Note that both "<<" and ">>" in Perl are implemented directly using "<<" and ">>" in C. If "use integer" (see "Integer Arithmetic") is in force then signed C integers are used, else unsigned C integers are used. Either way, the implementation isnt going to generate results larger than the size of the integer type Perl was built with (32 bits or 64 bits). The result of overflowing the range of the integers is undefined because it is undefined also in C. In other words, using 32-bit inte gers, "1 << 32" is undefined. Shifting by a negative number of bits is also undefined. Named Unary Operators The various named unary operators are treated as functions with one argument, with optional parentheses. If any list operator (print(), etc.) or any unary operator (chdir(), etc.) is followed by a left parenthesis as the next token, the opera tor and arguments within parentheses are taken to be of highest prece dence, just like a normal function call. For example, because named unary operators are higher precedence than ||: chdir \$foo || die; # (chdir \$foo) || die chdir(\$foo) || die; # (chdir \$foo) || die chdir (\$foo) || die; # (chdir \$foo) || die chdir +(\$foo) || die; # (chdir \$foo) || die but, because * is higher precedence than named operators: chdir \$foo * 20; # chdir (\$foo * 20) chdir(\$foo) * 20; # (chdir \$foo) * 20 chdir (\$foo) * 20; # (chdir \$foo) * 20 chdir +(\$foo) * 20; # chdir (\$foo * 20) rand 10 * 20; # rand (10 * 20) rand(10) * 20; # (rand 10) * 20 rand (10) * 20; # (rand 10) * 20 rand +(10) * 20; # rand (10 * 20) Regarding precedence, the filetest operators, like "-f", "-M", etc. are treated like named unary operators, but they dont follow this func tional parenthesis rule. That means, for example, that "-f(\$file).".bak"" is equivalent to "-f "\$file.bak"". See also "Terms and List Operators (Leftward)". Relational Operators Binary "<" returns true if the left argument is numerically less than the right argument. Binary ">" returns true if the left argument is numerically greater than the right argument. Binary "<=" returns true if the left argument is numerically less than or equal to the right argument. Binary ">=" returns true if the left argument is numerically greater than or equal to the right argument. 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. Equality Operators Binary "==" returns true if the left argument is numerically equal to the right argument. Binary "!=" returns true if the left argument is numerically not equal to the right argument. Binary "<=>" returns -1, 0, or 1 depending on whether the left argument is numerically less than, equal to, or greater than the right argument. If your platform supports NaNs (not-a-numbers) as numeric values, using them with "<=>" returns undef. NaN is not "<", "==", ">", "<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN returns true, as does NaN != anything else. If your platform doesnt support NaNs then NaN is just a string with numeric value 0. perl -le \$a = "NaN"; print "No NaN support here" if \$a == \$a perl -le \$a = "NaN"; print "NaN support here" if \$a != \$a Binary "eq" returns true if the left argument is stringwise 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. "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order speci fied by the current locale if "use locale" is in effect. See perllo cale. Bitwise And Binary "&" returns its operands ANDed together bit by bit. (See also "Integer Arithmetic" and "Bitwise String Operators".) Note that "&" has lower priority than relational operators, so for example the brackets are essential in a test like print "Even\n" if (\$x & 1) == 0; Bitwise Or and Exclusive Or Binary "|" returns its operands ORed together bit by bit. (See also "Integer Arithmetic" and "Bitwise String Operators".) Binary "^" returns its operands XORed together bit by bit. (See also "Integer Arithmetic" and "Bitwise String Operators".) Note that "|" and "^" have lower priority than relational operators, so for example the brackets are essential in a test like print "false\n" if (8 | 2) != 10; C-style Logical And Binary "&&" performs a short-circuit logical AND operation. 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. C-style Logical Or Binary "||" performs a short-circuit logical OR operation. That is, if the left operand is true, the right operand is not even evaluated. Scalar or list context propagates down to the right operand if it is evaluated. The "||" and "&&" operators return the last value evaluated (unlike Cs "||" and "&&", which return 0 or 1). Thus, a reasonably portable way to find out the home directory might be: \$home = \$ENV{HOME} || \$ENV{LOGDIR} || (getpwuid(\$<))[7] || die "Youre homeless!\n"; In particular, this means that you shouldnt use this for selecting between two aggregates for assignment: @a = @b || @c; # this is wrong @a = scalar(@b) || @c; # really meant this @a = @b ? @b : @c; # this works fine, though As more readable alternatives to "&&" and "||" when used for control flow, Perl provides "and" and "or" operators (see below). The short- circuit behavior is identical. The precedence of "and" and "or" is much lower, however, so that you can safely use them after a list oper ator without the need for parentheses: unlink "alpha", "beta", "gamma" or gripe(), next LINE; With the C-style operators that would have been written like this: unlink("alpha", "beta", "gamma") || (gripe(), next LINE); Using "or" for assignment is unlikely to do what you want; see below. Range Operators Binary ".." is the range operator, which is really two different opera tors depending on the context. 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 (1..10)" loops and for doing slice operations on arrays. In the current imple mentation, no temporary array is created when the range operator is used as the expression in "foreach" loops, but older versions of Perl might burn a lot of memory when you write something like this: for (1 .. 1_000_000) { # code } The range operator also works on strings, using the magical auto-incre ment, see below. In scalar context, ".." returns a boolean value. The operator is bistable, like a flip-flop, and emulates the line-range (comma) opera tor of sed, awk, and various editors. Each ".." operator maintains its own boolean state. 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 doesnt become false till the next time the range operator is evaluated. It can test the right operand and become false on the same evaluation it became true (as in awk), but it still returns true once. If you dont want it to test the right operand till the next evaluation, as in sed, just use three dots ("...") instead of two. In all other regards, "..." behaves just like ".." does. 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 precedence is a little lower than || and &&. 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. The final sequence number in a range has the string "E0" appended to it, which doesnt affect its numeric value, but gives you something to search for if you want to exclude the endpoint. You can exclude the beginning point by waiting for the sequence number to be greater than 1. If either operand of scalar ".." is a constant expression, that operand is considered true if it is equal ("==") to the current input line num ber (the \$. variable). To be pedantic, the comparison is actually "int(EXPR) == int(EXPR)", but that is only an issue if you use a floating point expression; when implicitly using \$. as described in the previous paragraph, the compar ison is "int(EXPR) == int(\$.)" which is only an issue when \$. is set to a floating point value and you are not reading from a file. Fur thermore, "span" .. "spat" or "2.18 .. 3.14" will not do what you want in scalar context because each of the operands are evaluated using their integer representation. Examples: As a scalar operator: if (101 .. 200) { print; } # print 2nd hundred lines, short for # if (\$. == 101 .. \$. == 200) ... next LINE if (1 .. /^\$/); # skip header lines, short for # ... if (\$. == 1 .. /^\$/); # (typically in a loop labeled LINE) s/^/> / if (/^\$/ .. eof()); # quote body # parse mail messages while (<>) { \$in_header = 1 .. /^\$/; \$in_body = /^\$/ .. eof; if (\$in_header) { # ... } else { # in body # ... } } continue { close ARGV if eof; # reset \$. each file } Heres a simple example to illustrate the difference between the two range operators: @lines = (" - Foo", "01 - Bar", "1 - Baz", " - Quux"); foreach (@lines) { if (/0/ .. /1/) { print "\$_\n"; } } This program will print only the line containing "Bar". If the range operator is changed to "...", it will also print the "Baz" line. And now some examples as a list operator: for (101 .. 200) { print; } # print \$_ 100 times @foo = @foo[0 .. \$#foo]; # an expensive no-op @foo = @foo[\$#foo-4 .. \$#foo]; # slice last 5 items The range operator (in list context) makes use of the magical auto- increment algorithm if the operands are strings. You can say @alphabet = (A .. Z); to get all normal letters of the English alphabet, or \$hexdigit = (0 .. 9, a .. f)[\$num & 15]; to get a hexadecimal digit, or @z2 = (01 .. 31); print \$z2[\$mday]; to get dates with leading zeros. 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 speci fied. Because each operand is evaluated in integer form, "2.18 .. 3.14" will return two elements in list context. @list = (2.18 .. 3.14); # same as @list = (2 .. 3); Conditional Operator Ternary "?:" is the conditional operator, just as in C. It works much like an if-then-else. If the argument before the ? is true, the argu ment before the : is returned, otherwise the argument after the : is returned. For example: printf "I have %d dog%s.\n", \$n, (\$n == 1) ? : "s"; Scalar or list context propagates downward into the 2nd or 3rd argu ment, whichever is selected. \$a = \$ok ? \$b : \$c; # get a scalar @a = \$ok ? @b : @c; # get an array \$a = \$ok ? @b : @c; # oops, thats just a count! The operator may be assigned to if both the 2nd and 3rd arguments are legal lvalues (meaning that you can assign to them): (\$a_or_b ? \$a : \$b) = \$c; Because this operator produces an assignable result, using assignments without parentheses will get you in trouble. For example, this: \$a % 2 ? \$a += 10 : \$a += 2 Really means this: ((\$a % 2) ? (\$a += 10) : \$a) += 2 Rather than this: (\$a % 2) ? (\$a += 10) : (\$a += 2) That should probably be written more simply as: \$a += (\$a % 2) ? 10 : 2; Assignment Operators "=" is the ordinary assignment operator. Assignment operators work as in C. That is, \$a += 2; is equivalent to \$a = \$a + 2; although without duplicating any side effects that dereferencing the lvalue might trigger, such as from tie(). Other assignment operators work similarly. The following are recognized: **= += *= &= <<= &&= -= /= |= >>= ||= .= %= ^= x= Although these are grouped by family, they all have the precedence of assignment. 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. This is useful for modi fying a copy of something, like this: (\$tmp = \$global) =~ tr [A-Z] [a-z]; Likewise, (\$a += 2) *= 3; is equivalent to \$a += 2; \$a *= 3; Similarly, a list assignment in list context produces the list of lval ues assigned to, and a list assignment in scalar context returns the number of elements produced by the expression on the right hand side of the assignment. Comma Operator Binary "," is the comma operator. In scalar context it evaluates its left argument, throws that value away, then evaluates its right argu ment and returns that value. This is just like Cs comma operator. In list context, its just the list argument separator, and inserts both its arguments into the list. The "=>" operator is a synonym for the comma, but forces any word (con sisting entirely of word characters) to its left to be interpreted as a string (as of 5.001). This includes words that might otherwise be con sidered a constant or function call. use constant FOO => "something"; my %h = ( FOO => 23 ); is equivalent to: my %h = ("FOO", 23); It is NOT: my %h = ("something", 23); If the argument on the left is not a word, it is first interpreted as an expression, and then the string value of that is used. The "=>" operator is helpful in documenting the correspondence between keys and values in hashes, and other paired elements in lists. %hash = ( \$key => \$value ); login( \$username => \$password ); List Operators (Rightward) On the right side of a list operator, it has very low precedence, such that it controls all comma-separated expressions found there. The only operators with lower precedence are the logical operators "and", "or", and "not", which may be used to evaluate calls to list operators with out the need for extra parentheses: open HANDLE, "filename" or die "Cant open: \$!\n"; See also discussion of list operators in "Terms and List Operators (Leftward)". Logical Not Unary "not" returns the logical negation of the expression to its right. Its the equivalent of "!" except for the very low precedence. Logical And Binary "and" returns the logical conjunction of the two surrounding expressions. Its equivalent to && except for the very low precedence. This means that it short-circuits: i.e., the right expression is evalu ated only if the left expression is true. Logical or and Exclusive Or Binary "or" returns the logical disjunction of the two surrounding expressions. Its equivalent to || except for the very low precedence. This makes it useful for control flow print FH \$data or die "Cant write to FH: \$!"; This means that it short-circuits: i.e., the right expression is evalu ated only if the left expression is false. Due to its precedence, you should probably avoid using this for assignment, only for control flow. \$a = \$b or \$c; # bug: this is wrong (\$a = \$b) or \$c; # really means this \$a = \$b || \$c; # better written this way However, when its a list-context assignment and youre trying to use "||" for control flow, you probably need "or" so that the assignment takes higher precedence. @info = stat(\$file) || die; # oops, scalar sense of stat! @info = stat(\$file) or die; # better, now @info gets its due Then again, you could always use parentheses. Binary "xor" returns the exclusive-OR of the two surrounding expres sions. It cannot short circuit, of course. C Operators Missing From Perl Here is what C has that Perl doesnt: unary & Address-of operator. (But see the "\" operator for taking a reference.) unary * Dereference-address operator. (Perls prefix dereferencing operators are typed: \$, @, %, and &.) (TYPE) Type-casting operator. Quote and Quote-like Operators While we usually think of quotes as literal values, in Perl they func tion as operators, providing various kinds of interpolating and pattern matching capabilities. Perl provides customary quote characters for these behaviors, but also provides a way for you to choose your quote character for any of them. In the following table, a "{}" represents any pair of delimiters you choose. Customary Generic Meaning Interpolates q{} Literal no "" qq{} Literal yes qx{} Command yes* qw{} Word list no // m{} Pattern match yes* qr{} Pattern yes* s{}{} Substitution yes* tr{}{} Transliteration no (but see below) <{key}[0]" are also interpolated, as are array and hash slices. But method calls such as "\$obj->meth" are not. Interpolating an array or slice interpolates the elements in order, separated by the value of \$", so is equivalent to interpolating "join \$", @array". "Punctuation" arrays such as "@+" are only interpolated if the name is enclosed in braces "@{+}". You cannot include a literal "\$" or "@" within a "\Q" sequence. An unescaped "\$" or "@" interpolates the corresponding variable, while escaping will cause the literal string "\\$" to be inserted. Youll need to write something like "m/\Quser\E\@\Qhost/". Patterns are subject to an additional level of interpretation as a reg ular 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. If this is not what you want, use "\Q" to interpolate a variable literally. 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. Regexp Quote-Like Operators Here are the quote-like operators that apply to pattern matching and related activities. ?PATTERN? This is just like the "/pattern/" search, except that it matches only once between calls to the reset() operator. 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. Only "??" patterns local to the current package are reset. while (<>) { if (?^\$?) { # blank line between header and body } } continue { reset if eof; # clear ?? status for next file } This usage is vaguely deprecated, which means it just might possibly be removed in some distant future version of Perl, perhaps somewhere around the year 2168. m/PATTERN/cgimosx /PATTERN/cgimosx Searches a string for a pattern match, and in scalar context returns true if it succeeds, false if it fails. If no string is specified via the "=~" or "!~" operator, the \$_ string is searched. (The string specified with "=~" need not be an lvalue--it may be the result of an expression evaluation, but remember the "=~" binds rather tightly.) See also perlre. See perllocale for discussion of additional considerations that apply when "use locale" is in effect. Options are: c Do not reset search position on a failed match when /g is in effect. g Match globally, i.e., find all occurrences. i Do case-insensitive pattern matching. m Treat string as multiple lines. o Compile pattern only once. s Treat string as single line. x Use extended regular expressions. If "/" is the delimiter then the initial "m" is optional. With the "m" you can use any pair of non-alphanumeric, non-whites pace characters as delimiters. This is particularly useful for matching path names that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is the delimiter, then the match- only-once rule of "?PATTERN?" applies. If "" is the delim iter, no interpolation is performed on the PATTERN. PATTERN may contain variables, which will be interpolated (and the pattern recompiled) every time the pattern search is evalu ated, except for when the delimiter is a single quote. (Note that \$(, \$), and \$| are not interpolated because they look like end-of-string tests.) If you want such a pattern to be com piled only once, add a "/o" after the trailing delimiter. This avoids expensive run-time recompilations, and is useful when the value you are interpolating wont change over the life of the script. However, mentioning "/o" constitutes a promise that you wont change the variables in the pattern. If you change them, Perl wont even notice. See also "qr/STRING/imosx". If the PATTERN evaluates to the empty string, the last success fully matched regular expression is used instead. In this case, only the "g" and "c" flags on the empty pattern is honoured - 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). If the "/g" option is not used, "m//" in list context returns a list consisting of the subexpressions matched by the parenthe ses in the pattern, i.e., (\$1, \$2, \$3...). (Note that here \$1 etc. are also set, and that this differs from Perl 4s behav ior.) When there are no parentheses in the pattern, the return value is the list "(1)" for success. With or without parenthe ses, an empty list is returned upon failure. Examples: open(TTY, /dev/tty); =~ /^y/i && foo(); # do foo if desired if (/Version: *([0-9.]*)/) { \$version = \$1; } next if m#^/usr/spool/uucp#; # poor mans grep \$arg = shift; while (<>) { print if /\$arg/o; # compile only once } if ((\$F1, \$F2, \$Etc) = (\$foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) This last example splits \$foo into the first two words and the remainder of the line, and assigns those three fields to \$F1, \$F2, and \$Etc. The conditional is true if any variables were assigned, i.e., if the pattern matched. The "/g" modifier specifies global pattern matching--that is, matching as many times as possible within the string. 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. In scalar context, each execution of "m//g" finds the next match, returning true if it matches, and false if there is no further match. The position after the last match can be read or set using the pos() function; see "pos" in perlfunc. A failed match normally resets the search position to the begin ning of the string, but you can avoid that by adding the "/c" modifier (e.g. "m//gc"). Modifying the target string also resets the search position. You can intermix "m//g" matches with "m/\G.../g", where "\G" is a zero-width assertion that matches the exact position where the previous "m//g", if any, left off. Without the "/g" modi fier, the "\G" assertion still anchors at pos(), but the match is of course only attempted once. Using "\G" without "/g" on a target string that has not previously had a "/g" match applied to it is the same as using the "\A" assertion to match the beginning of the string. Note also that, currently, "\G" is only properly supported when anchored at the very beginning of the pattern. Examples: # list context (\$one,\$five,\$fifteen) = (uptime =~ /(\d+\.\d+)/g); # scalar context \$/ = ""; while (defined(\$paragraph = <>)) { while (\$paragraph =~ /[a-z][")]*[.!?]+[")]*\s/g) { \$sentences++; } } print "\$sentences\n"; # using m//gc with \G \$_ = "ppooqppqq"; while (\$i++ < 2) { print "1: "; print \$1 while /(o)/gc; print ", pos=", pos, "\n"; print "2: "; print \$1 if /\G(q)/gc; print ", pos=", pos, "\n"; print "3: "; print \$1 while /(p)/gc; print ", pos=", pos, "\n"; } print "Final: \$1, pos=",pos,"\n" if /\G(.)/; The last example should print: 1: oo, pos=4 2: q, pos=5 3: pp, pos=7 1: , pos=7 2: q, pos=8 3: , pos=8 Final: q, pos=8 Notice that the final match matched "q" instead of "p", which a match without the "\G" anchor would have done. Also note that the final match did not update "pos" -- "pos" is only updated on a "/g" match. If the final match did indeed match "p", its a good bet that youre running an older (pre-5.6.0) Perl. A useful idiom for "lex"-like scanners is "/\G.../gc". You can combine several regexps like this to process a string part-by-part, doing different actions depending on which regexp matched. Each regexp tries to match where the previous one leaves off. \$_ = <&1; To capture a commands STDOUT but discard its STDERR: \$output = cmd 2>/dev/null; To capture a commands STDERR but discard its STDOUT (ordering is important here): \$output = cmd 2>&1 1>/dev/null; To exchange a commands STDOUT and STDERR in order to capture the STDERR but leave its STDOUT to come out the old STDERR: \$output = cmd 3>&1 1>&2 2>&3 3>&-; To read both a commands STDOUT and its STDERR separately, its easiest to redirect them separately to files, and then read from those files when the program is done: system("program args 1>program.stdout 2>program.stderr"); Using single-quote as a delimiter protects the command from Perls double-quote interpolation, passing it on to the shell instead: \$perl_info = qx(ps \$\$); # thats Perls \$\$ \$shell_info = qxps \$\$; # thats the new shells \$\$ How that string gets evaluated is entirely subject to the com mand interpreter on your system. On most platforms, you will have to protect shell metacharacters if you want them treated literally. This is in practice difficult to do, as its unclear how to escape which characters. See perlsec for a clean and safe example of a manual fork() and exec() to emulate backticks safely. On some platforms (notably DOS-like ones), the shell may not be capable of dealing with multiline commands, so putting newlines in the string may not get you what you want. You may be able to evaluate multiple commands in a single line by separating them with the command separator character, if your shell sup ports that (e.g. ";" on many Unix shells; "&" on the Windows NT "cmd" shell). Beginning with v5.6.0, Perl will attempt to flush all files opened for output before starting the child process, but this may not be supported on some platforms (see perlport). To be safe, you may need to set \$| (\$AUTOFLUSH in English) or call the "autoflush()" method of "IO::Handle" on any open handles. Beware that some command shells may place restrictions on the length of the command line. You must ensure your strings dont exceed this limit after any necessary interpolations. 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. That doesnt mean you should go out of your way to avoid backticks when theyre the right way to get something done. Perl was made to be a glue language, and one of the things it glues together is commands. Just under stand what youre getting yourself into. See "I/O Operators" for more discussion. qw/STRING/ Evaluates to a list of the words extracted out of STRING, using embedded whitespace as the word delimiters. It can be under stood as being roughly equivalent to: split( , q/STRING/); the differences being that it generates a real list at compile time, and in scalar context it returns the last element in the list. So this expression: qw(foo bar baz) is semantically equivalent to the list: foo, bar, baz Some frequently seen examples: use POSIX qw( setlocale localeconv ) @EXPORT = qw( foo bar baz ); A common mistake is to try to separate the words with comma or to put comments into a multi-line "qw"-string. For this rea son, the "use warnings" pragma and the -w switch (that is, the \$^W variable) produces warnings if the STRING contains the "," or the "#" character. s/PATTERN/REPLACEMENT/egimosx 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). If no string is specified via the "=~" or "!~" operator, the \$_ variable is searched and modified. (The string specified with "=~" 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 a single quote, no 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 at run-time. If you want the pattern compiled only once the first time the variable is interpolated, use the "/o" option. If the pattern evaluates to the empty string, the last successfully executed regular expression is used instead. See perlre for further explanation on these. See perllocale for discussion of additional considerations that apply when "use locale" is in effect. Options are: e Evaluate the right side as an expression. g Replace globally, i.e., all occurrences. i Do case-insensitive pattern matching. m Treat string as multiple lines. o Compile pattern only once. s Treat string as single line. x Use extended regular expressions. Any non-alphanumeric, non-whitespace delimiter may replace the slashes. If single quotes are used, no interpretation is done on the replacement string (the "/e" modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks as normal delimiters; the replacement text is not evaluated as a command. If the PATTERN is delimited by bracketing quotes, the REPLACE MENT has its own pair of quotes, which may or may not be brack eting quotes, e.g., "s(foo)(bar)" or "s/bar/". A "/e" will cause the replacement portion to be treated as a full- fledged Perl expression and evaluated right then and there. It is, however, syntax checked at compile-time. A second "e" modi fier will cause the replacement portion to be "eval"ed before being run as a Perl expression. Examples: s/\bgreen\b/mauve/g; # dont change wintergreen \$path =~ s|/usr/bin|/usr/local/bin|; s/Login: \$foo/Login: \$bar/; # run-time pattern (\$foo = \$bar) =~ s/this/that/; # copy first, then change \$count = (\$paragraph =~ s/Mister\b/Mr./g); # get change-count \$_ = abc123xyz; s/\d+/\$&*2/e; # yields abc246xyz s/\d+/sprintf("%5d",\$&)/e; # yields abc 246xyz s/\w/\$& x 2/eg; # yields aabbcc 224466xxyyzz s/%(.)/\$percent{\$1}/g; # change percent escapes; no /e s/%(.)/\$percent{\$1} || \$&/ge; # expr now, so /e s/^=(\w+)/&pod(\$1)/ge; # use function call # expand variables in \$_, but dynamics only, using # symbolic dereferencing s/\\$(\w+)/\${\$1}/g; # Add one to the value of any numbers in the string s/(\d+)/1 + \$1/eg; # This will expand any embedded scalar variable # (including lexicals) in \$_ : First \$1 is interpolated # to the variable name, and then evaluated s/(\\$\w+)/\$1/eeg; # Delete (most) C comments. \$program =~ s { /\* # Match the opening delimiter. .*? # Match a minimal number of characters. \*/ # Match the closing delimiter. } []gsx; s/^\s*(.*?)\s*\$/\$1/; # trim whitespace in \$_, expensively for (\$variable) { # trim whitespace in \$variable, cheap s/^\s+//; s/\s+\$//; } s/([^ ]*) *([^ ]*)/\$2 \$1/; # reverse 1st two fields Note the use of \$ instead of \ in the last example. Unlike sed, we use the \ form in only the left hand side. Any where else its \$. Occasionally, you cant use just a "/g" to get all the changes to occur that you might want. Here are two common cases: # put commas in the right places in an integer 1 while s/(\d)(\d\d\d)(?!\d)/\$1,\$2/g; # expand tabs to 8-column spacing 1 while s/\t+/ x (length(\$&)*8 - length(\$)%8)/e; tr/SEARCHLIST/REPLACEMENTLIST/cds y/SEARCHLIST/REPLACEMENTLIST/cds 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. 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 "tr/A-J/0-9/" does the same replacement as "tr/ACEG IBDFHJ/0246813579/". For sed devotees, "y" is provided as a synonym for "tr". If the SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has its own pair of quotes, which may or may not be bracketing quotes, e.g., "tr[A-Z][a-z]" or "tr(+\-*/)/ABCD/". Note that "tr" does not do regular expression character classes such as "\d" or "[:lower:]". The ```