Sorry if this is duplicated. I thought I'd reword my question a little bit.
How could I use regex to evaluate a mathematical expression? Without using the eval function.
Example expressions:
math1 = "1+1"
math2 = "3+2-1"
I would like it to work for a variable number of numbers in the expression like I showed in the example.
This is just a bad idea. Regexp is not a parser, nor an evaluator.
Use a grammar to describe your expressions. Parse it with a formal parser like the lovely ruby gem Treetop. Then evaluate the abstract syntax tree (AST) produced by the parser.
Gosh, Treetop's arithmetic example practically gives you the solution for free.
This is a little late, but I wrote a gem for evaluating arbitrary mathematical expressions (and it doesn't use eval internally): https://github.com/rubysolo/dentaku
For addition and subtraction, this should work
(?:(/d+)([-+]))+(/d+)
This means:
Note that each individual number and sign are captured in groups 1..n
So to evaluate, you could take captures 1 and 3, applying the sign from capture 2. Then apply the sign from capture 4 (if it exists) with the previous result and the number from capture 5 (which must exist if capture 4 exists) and so on...
So to evaluate, in psuedo code:
i=1
result=capture(i)
loop while i <= (n-2) (where n is the capture count):
If capture(i+1) == "-" // is subtraction
result = result - capture(i+2)
Else // is addition
result = result + capture(i+2)
End if
i = i + 2
End while
This is only going to work for simple addition and subtraction like in the examples you provided, as it relies on left to right associativity. As others have suggested, you'll probably need to properly parse anything more complex, eg by building a tree of nodes that can then be evaluated in the correct (depth-first?) order.
This is really messy…
math2 = "12+3-4"
head, *tail = math2.scan(/(?<digits>\d+)(?<op>[\+\-\*\/])?/)
.map{|(digits,op)|
[digits.to_i,op]
}
.reverse
tail.inject(head.first){|sum,(digits,op)|
op.nil? ?
digits :
digits.send(op,sum)
}
# => 11
You should really consider a parser though.
send, is that allowed?