Regex replace everything not starting with multiple times in line

Question

I am making a parser of sorts in javascript that takes a mathematical expression given to the script as a string, and evaluates it and does some other things with it. If the users want to use builtin Javascript mathematical functions, they have to enter the following string e.g. "1 + Math.log(x)". That becomes very tedious when things get nested e.g "Math.abs(Math.log(Math.pow(x, 2))) + Math.log2(x)". As you can see, the "Math." part of it not only takes longer to write, but makes it less readable. I want to remove that "Math." part. The way I've done it is using simple regex that basically has a list of all Javascript Math constants and methods and simply prepends the "Math." part of it. Simple enough:

  input = input.replace(/(E|PI|SQRT2|SQRT1_2|LN2|LN10|LOG2E|LOG10E)/g, "Math.$1");

The same things happens for the methods. This works fine. But as always that's not very flexible and leaves room for misunderstanding and somebody may coma along and insist on typing Math.log(x) which will in turn be replaced with Math.Math.log(x), which won't work.

What I want to know is, is there some way to match any of these predefined strings (constants and methods) so that they will only be matched via regex if they don't have the "Math." part in front of it. I have tried this

^(?!Math\.)(log2|log|exp|abs)

but it is quite useless, as this doesn't work with nesting and even multiple operands. Is there any way to do this purely in regex, as this would make the entire process more elegant. Any help would be appreciated.

Answer 1

You can use the following trick so that it gets replaced even if it matches or not:

(?:Math\.)?(log2|log|exp|abs|pow)

And replace with Math.$1

See DEMO