How do I ensure that a regex does not match an empty string?
- by Dancrumb
I'm using the Jison parser generator for Javascript and am having problems with my language specification.
The program I'm writing will be a calculator that can handle feet, inches and sixteenths.
In order to do this, I have the following specification:
%%
([0-9]+\s*"'")?\s*([0-9]+\s*"\"")?\s*([0-9]+\s*"s")? {return 'FIS';}
[0-9]+("."[0-9]+)?\b {return 'NUMBER';}
\s+ {/* skip whitespace */}
"*" {return '*';}
"/" {return '/';}
"-" {return '-';}
"+" {return '+';}
"(" {return '(';}
")" {return ')';}
<<EOF>> {return 'EOF';}
Most of these lines come from a basic calculator specification. I simply added the first line.
The regex correctly matches feet, inch, sixteenths, such as 6'4" (six feet, 4 inches) or 4"5s (4 inches, 5 sixteenths) with any kind of whitespace between the numbers and indicators.
The problem is that the regex also matches a null string. As a result, the lexical analysis always records a FIS at the start of the line and then the parsing fails.
Here is my question: is there a way to modify this regex to guarantee that it will only match a non-zero length string?
