=~

MatchâMatches rxp against str.

/at/ =~ "input data"   #=> 7
/ax/ =~ "input data"   #=> nil

If =~ is used with a regexp literal with named captures, captured strings (or nil) is assigned to local variables named by the capture names.

/(?<lhs>\w+)\s*=\s*(?<rhs>\w+)/ =~ "  x = y  "
p lhs    #=> "x"
p rhs    #=> "y"

If it is not matched, nil is assigned for the variables.

/(?<lhs>\w+)\s*=\s*(?<rhs>\w+)/ =~ "  x = "
p lhs    #=> nil
p rhs    #=> nil

This assignment is implemented in the Ruby parser. The parser detects 'regexp-literal =~ expression' for the assignment. The regexp must be a literal without interpolation and placed at left hand side.

The assignment does not occur if the regexp is not a literal.

re = /(?<lhs>\w+)\s*=\s*(?<rhs>\w+)/
re =~ "  x = y  "
p lhs    # undefined local variable
p rhs    # undefined local variable

A regexp interpolation, #{}, also disables the assignment.

rhs_pat = /(?<rhs>\w+)/
/(?<lhs>\w+)\s*=\s*#{rhs_pat}/ =~ "x = y"
p lhs    # undefined local variable

The assignment does not occur if the regexp is placed at the right hand side.

"  x = y  " =~ /(?<lhs>\w+)\s*=\s*(?<rhs>\w+)/
p lhs, rhs # undefined local variable