To test which mathematical operator (*, /, +, -) exists in the following items:
13*4
7+8
-15/3
7-5
I have the following regex for JavaScript:
/[\*\+/\-]/
The problem is that regex will identify the negative sign in -15/3 as an operator. How do I make it NOT match on first character in the string?
You can match a regex like this:
One or more digits (also allowing a decimal)
followed by optional whitespace
followed by an operator
followed by optional whitespace
followed by one or more digits (also allowing a decimal)
and then reference the captured group in the middle to get just the operator:
/[\d.]+\s*([*+\-\/])\s*[\d.]+/
Sample code:
var match = str.match(/[\d.]+\s*([*+\-\/])\s*[\d.]+/);
if (match) {
var operator = match[1];
}
Working demo and test code: http://jsfiddle.net/jfriend00/DgrSg/
If you want to allow more than just digits before the operators (such as other expressions), then you will have to do a lot more than a simple or even complex regex. This answer assumes you're just trying to solve the examples that you've presented.
You could use:
/.[\*\+/\-]/
which just ensures there's a character in front of the symbol.
That's a pretty quick and dirty fix, I'd probably prefer to make a full expression evaluator to catch the hundred other potential error conditions but it'll do if you want something simple and only marginally more robust than what you have.
//d[\*\+/\-]/ ?
\d rather than /d. But you should really consider the idea of a proper expression evaluator since a regex doesn't give you the full power - there'll almost always be more edge cases you have to worry about :-)Check that your operator is preceded by a digit and followed by either a digit or a minus sign and a digit.
/\d[\*\+/\-]-?\d/
5--5.