Perl 匹配运算符 =~

Question

This is a very basic question but I am not able to find any proper documentation explaining this behaviour.

("Johnson" =~ /son/ ) returns true but (/son/ =~ "Johnson") returns false. What is the exact reason? Why =~ operator behaves differently when the operands are interchanged?

Answer 1

STRAIGHT OUTTA DOCS:

The simplest regexp is simply a word, or more generally, a string of characters. A regexp consisting of a word matches any string that contains that word:

"Hello World" =~ /World/;  # matches 

What is this Perl statement all about? "Hello World" is a simple double-quoted string. World is the regular expression and the // enclosing /World/ tells Perl to search a string for a match. The operator =~ associates the string with the regexp match and produces a true value if the regexp matched , or false if the regexp did not match. In our case, World matches the second word in "Hello World" , so the expression is true.

Please read http://perldoc.perl.org/perlretut.html

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Now in your example "Johnson" =~ /son/ matches because RHS of =~ (which is son) is found in LHS (Johnson). In case of /son/ =~ "Johnson" RHS (Johnson) is not found in LHS (son).

Answer 2

Well... because the =~ operator binds a scalar expression, to a pattern match.

So it states the order in which the arguments need to be given. Your second (/son/ =~ "Johnson") uses Johnson as pattern... and that one is not hit, thus false.

See binding operators: https://users.cs.cf.ac.uk/Dave.Marshall/PERL/node87.html