Separating the numbers, special characters and + or - sign from a string in python

Question

So, I have a strings like

a = ';1'

b = '2+3'

c = '32'

d = '12-'

e = '2+;'

f = '2'

I want to separate them to get the following results:

a: [';', '1']

b: ['2+', '3']

c: ['3', '2']

d: ['1', '2-']

e: ['2+', ';']

`f: ['2', None]

The + or - sign always come after the digit.

Answer 1

You can do something of this sort, this works fine for all the inputs you have provided. Not the most pythonic way to do it, but it works.

a = ';1'
b = '2+3'
c = '32'
d = '12-'
e = '2+;'

inputs = [a, b, c, d, e]
output = list()
for expr in inputs:
    i = 0
    string = str()
    li = list()
    while (i < len(expr)):
        if (expr[i] >= '0' and expr[i] <= '9') and i < len(expr) - 1:
            if expr[i + 1] == '+' or expr[i + 1] == '-':
                string += expr[i]
                string += expr[i + 1]
                li.append(string)
                i += 1
            else:
                li.append(expr[i])
        else:
            li.append(expr[i])
        i += 1
    output.append(li)

print(output)

Answer 2

Using pairwise() and chain() from the built-in itertools module:

# itertools.pairwise is available on python 3.10+, use this function on earlier versions
# copied from https://docs.python.org/3/library/itertools.html#itertools.pairwise
def pairwise(iterable):
    # pairwise('ABCDEFG') --> AB BC CD DE EF FG
    a, b = tee(iterable)
    next(b, None)
    return zip(a, b)

def split(s: str) -> typing.List[str]:
    # iterate pairwise: "2+3" --> [("2", "+"), ("+", "3"), ("3", None)]
    # the None is just added to get the last number right
    pairs = pairwise(chain(s, [None]))
    output = []
    for l in pairs:
        if l[1] and l[1] in "+-":
            # number is followed by a "+" or "-" --> join together
            output.append("".join(l))
        elif l[0] not in "+-":
            # number is not followed by +/-, append the number
            output.append(l[0])
    return output

# checking your examples:
strings = [';1', '2+3', '32', '12-', '2+;']
[split(s) for s in strings]
# [[';', '1'], ['2+', '3'], ['3', '2'], ['1', '2-'], ['2+', ';']]

# bonus: everything in one line because why not
[[("".join(l) if l[1] and l[1] in "+-" else l[0]) for l in pairwise(chain(s, [None])) if l[0] not in "+-"] for s in strings]
# [[';', '1'], ['2+', '3'], ['3', '2'], ['1', '2-'], ['2+', ';']]