How to include/exclude non-digits inside a string of numbers

Question

Im trying to make a quadratic calculator for problems that follow ax^2 + bx + c = 0, but i want to include certain symbols (that being - in this case for if a number is negative) but exclude numbers followed by a certain symbol (that being ^ followed by 1 number afterwards (^2 for this case)). Ive already tried adding a second if statement and if/else statement. If it matters the code i am currently running is:

problem = input("give quadratic problem ")

new_string = ("{0}".format(problem))
 
emp_str = ""
for m in new_string:
    if m.isdigit():
        emp_str = emp_str + m
print(emp_str) 

print("The problem is {0}".format(problem))

im not very good with coding generally so there is a very big possibility there is something basic i am missing but any help is appriciated

Answer 1

This is easy to achieve using the re module(RegEx). Code:

import re
a = input("give quadratic problem ")
a = ' '+a
crit = re.compile(r"[^^](-\s*\d+\.?\d*|\d+\.?\d*)")
resarr = re.findall(crit, a)
resarr=[float(i.replace(' ', '')) for i in resarr]
print(resarr)

Output:

give quadratic problem 1.2x^2 - 2x - 3 = 0
[1.2, -2.0, -3.0, 0.0]

Edit:

Explaination of RegEx:

"[^^](-\s*\d+|\d+)"
[^^] -> rejects the match if it is preceded by a ^ sign to rule out exponents
Conditions:

  1. Negative numbers: -\s*\d+\.?\d* 
     First match the - sign
     Spaces in order to match even if - and the number have a space
     \d+ for the actual number
  2. Positive numbers: \d+\.?\d*
     Matches one or more successive digits(and a decimal followed by digits)
Note that the decimal is matched 0 or 1 times

Answer 2

You could use a regular expression with a negative lookbehind.

>>> expression = "1.2x^2 - 2x - 3 = 0"
>>> nums = re.findall(r"(?<!\^)(-?\s*\d+\.?\d*)", expression)
['1.2', '- 2', '- 3', ' 0']

Since this regex also includes a pattern for decimal points, you can also capture coefficients that aren't integers. Then, take out the spaces in each string, and convert it to a float:

>>> nums = [float(x.replace(' ', '')) for x in nums]
[1.2, -2.0, -3.0, 0.0]

Explanation of the regex (Try online):

(?<!\^)(-?\s*\d+\.?\d*)

(?<!  )                           : Negative lookbehind, make sure the string in the lookbehind doesn't exist before the match
    \^                            : This string is a caret
       (              )           : Capturing group
        -?\s*                     : Zero or one hyphens, followed by any number of whitespace...
              \d+\.?\d*           : ... followed by one or more digits, then an optional decimal point, then zero or more digits