I've just learned regex in python3 and was trying to solve a problem. The problem is something like this:

You have given a string where the first part is a float or integer number and the next part is a substring. You must split the number and the substring and return it as a list. The substring will only contain the alphabet from a-z and A-Z. The values of numbers can be negative. For example:

1. Input: 2.5ax
   Output:['2.5','ax']
2. Input: -5bcf
   Output:['-5','bcf']
3. Input:-69.67Gh
   Output:['-69.67','Gh']

and so on.

I did several attempts with regex to solve the problem.

1st attempt:

import re
i=input()
print(re.findall(r'^(-?\d (\.\d )?)|[a-zA-Z] $',i))

For the input -2.55xy, the expected output was ['-2.55','xy'] But the output came:

[('-2.55', '.55'), ('', '')]

2nd attempt: My second attempt was similar to my first attempt just a little different:

import re
i=input()
print(re.findall(r'^(-?(\d \.\d )|\d )|[a-zA-Z] $',i))

For the same input -2.55xy,the output came:

[('-2.55', '2.55'), ('', '')]

3rd attempt: My next attempt was like that:

import re
i=input()
print(re.findall(r'^-?[1-9.] |[a-z|A-Z] $',i))

which matched the expected output for -2.55xy and also with the sample examples. But when the input is 2..5 or something like that, it considers that also as a float.

4th attempt:

import re
i=input()
value=re.findall(r"[a-zA-Z] ",i)
print([i.replace(value[0],""),value[0]])

which also matches with the expected output but the same problem as 3rd one goes with it. Also, it doesn't look like an effective way to do it.

Conclusion: So I don't know why my 1st and 2nd attempt isn't working. The output comes with a list of tuples which is maybe because of the groups but I don't know the exact reason and don't really know how to solve them. Maybe I didn't understand the way the pattern works. Also why the substring didn't show in the output? In the end, I want to know what's the mistake in my code and how can I write better and more efficient code to solve the problem. Thank you and sorry for my bad English.

CodePudding user response：

The alternation | matches either the left part or the right part.

If the chars a-zA-Z are after the digit, you don't need the alternation | and you can use 2 capture groups to get the matches in that order.

Then using re.findall will return a list of tuples for the capture group values.

(-?\d (?:\.\d )?)([a-zA-Z] )

Explanation

• ( Capture group 1
  • -?\d Match an optional -
  • (?:\.\d )? Optionally match . and 1 digits using a non capture group (so it is not outputted separately by re.findall)
• ) Close group 1
• ( Capture group 2
  • [a-zA-Z] Match 1 times a char a-z or A-Z
• ) Close group 2

regex demo

import re

strings = [
    "2.5ax",
    "-5bcf",
    "-69.67Gh",
]

pattern = r"(-?\d (?:\.\d )?)([a-zA-Z] )"
for s in strings:
    print(re.findall(pattern, s))

Output

[('2.5', 'ax')]
[('-5', 'bcf')]
[('-69.67', 'Gh')]

CodePudding user response：

lookahead and lookbehind in re.sub simplify things sometimes.

• (?<=\d) look behind
• (?=[a-zA-Z]) look ahead

that is split between the digit and the letter.

strings = [
    "2.5ax",
    "-5bcf",
    "-69.67Gh",
]

for s in strings:
    print(re.split(r'(?<=\d)(?=[a-zA-Z])', s))


['2.5', 'ax']
['-5', 'bcf']
['-69.67', 'Gh']

CodePudding user response：

If the right side of the string are characters from a-z or A-Z then this method works:

string = input("Input: ")
left_side = ''
right_side = ''

for i in string:
    if i.isalpha():
        right_side  = i
    else:
        left_side  = i

print("Output:", [left_side, right_side])

Example:

Input: 2.5ax
Output: ['2.5', 'ax']

Based on what I understand from your question this can be done without using regex. Please tell me if I misunderstood your question.