How do I compare two strings in Perl?

How do I compare two strings in Perl?

See perldoc perlop. Use lt, gt, eq, ne, and cmp as appropriate for string comparisons:

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.

Binary ~~ does a smartmatch between its arguments. …

lt, le, ge, gt and cmp use the collation (sort) order specified by the current locale if a legacy use locale (but not use locale :not_characters) is in effect. See perllocale. Do not mix these with Unicode, only with legacy binary encodings. The standard Unicode::Collate and Unicode::Collate::Locale modules offer much more powerful solutions to collation issues.

  • cmp Compare

    a cmp b # -1
    b cmp a #  1
    a cmp a #  0
    
  • eq Equal to

    a eq  b #  0
    b eq  a #  0
    a eq  a #  1
    
  • ne Not-Equal to

    a ne  b #  1
    b ne  a #  1
    a ne  a #  0
    
  • lt Less than

    a lt  b #  1
    b lt  a #  0
    a lt  a #  0
    
  • le Less than or equal to

    a le  b #  1
    b le  a #  0
    a le  a #  1
    
  • gt Greater than

    a gt  b #  0
    b gt  a #  1
    a gt  a #  0
    
  • ge Greater than or equal to

    a ge  b #  0
    b ge  a #  1
    a ge  a #  1
    

See perldoc perlop for more information.

( Im simplifying this a little bit as all but cmp return a value that is both an empty string, and a numerically zero value instead of 0, and a value that is both the string 1 and the numeric value 1. These are the same values you will always get from boolean operators in Perl. You should really only be using the return values for boolean or numeric operations, in which case the difference doesnt really matter. )

How do I compare two strings in Perl?

In addtion to Sinan Ünür comprehensive listing of string comparison operators, Perl 5.10 adds the smart match operator.

The smart match operator compares two items based on their type. See the chart below for the 5.10 behavior (I believe this behavior is changing slightly in 5.10.1):

perldoc perlsyn Smart matching in detail:

The behaviour of a smart match depends on what type of thing its arguments are. It is always commutative, i.e. $a ~~ $b behaves the same as $b ~~ $a . The behaviour is determined by the following table: the first row that applies, in either order, determines the match behaviour.

  $a      $b        Type of Match Implied    Matching Code
  ======  =====     =====================    =============
  (overloading trumps everything)

  Code[+] Code[+]   referential equality     $a == $b   
  Any     Code[+]   scalar sub truth         $b−>($a)   

  Hash    Hash      hash keys identical      [sort keys %$a]~~[sort keys %$b]
  Hash    Array     hash slice existence     grep {exists $a−>{$_}} @$b
  Hash    Regex     hash key grep            grep /$b/, keys %$a
  Hash    Any       hash entry existence     exists $a−>{$b}

  Array   Array     arrays are identical[*]
  Array   Regex     array grep               grep /$b/, @$a
  Array   Num       array contains number    grep $_ == $b, @$a 
  Array   Any       array contains string    grep $_ eq $b, @$a 

  Any     undef     undefined                !defined $a
  Any     Regex     pattern match            $a =~ /$b/ 
  Code()  Code()    results are equal        $a−>() eq $b−>()
  Any     Code()    simple closure truth     $b−>() # ignoring $a
  Num     numish[!] numeric equality         $a == $b   
  Any     Str       string equality          $a eq $b   
  Any     Num       numeric equality         $a == $b   

  Any     Any       string equality          $a eq $b   

+ − this must be a code reference whose prototype (if present) is not 
(subs with a  prototype are dealt with by the Code() entry lower down) 
* − that is, each element matches the element of same index in the other
array. If a circular reference is found, we fall back to referential 
equality.   
! − either a real number, or a string that looks like a number

The matching code doesnt represent the real matching code, of course: its just there to explain the intended meaning. Unlike grep, the smart match operator will short-circuit whenever it can.

Custom matching via overloading
You can change the way that an object is matched by overloading the ~~ operator. This trumps the usual smart match semantics. See overload.

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