Regex to only allow leading zeros if it contains a decimal-CodePudding

I have a regex that looks like this:

bool(re.match('^(0|[.]{0,1}[1-9][.]{0,1}[0-9]*)$', "0.123"))

This works perfect, but it doesn't match for leading zeros when there contains a decimal like the above example. My objective is to only match leading zero numbers when they contain a decimal. Strings like: "01" should not be matched. Numbers that are decimal numbers or whole numbers can be matched.

How would I create a regex that matches for this?

Some scenarios:

0.01 Match
1 Match
1.2 Match
.01 Match
-0.1 Match
-1 Match
1.2.3 No Match
01 No Match
001.1 No Match
00.1 No Match

CodePudding user response：

Here's a non-regex approach which looks like passes all the mentioned test cases:

numbers = """\
0.123
0
01
0.01
1
1.2
.01
-0.1
-1
1.2.3
-01.2
01
-01
001.1
00.1\
"""


def is_valid_num(s: str):
    unsigned_s = s.lstrip('- ')

    if unsigned_s.startswith('0'):
        try:
            if not unsigned_s[1] == '.':
                return False
        except IndexError:
            # It's just a zero (0)
            return True

    try:
        _ = float(s)
        return True
    except ValueError:
        return False


if __name__ == '__main__':
    for n in numbers.split('\n'):
        print(f'{n:<10} -> {is_valid_num(n)!r:>10}')

Output:

0.123      ->       True
0          ->       True
01         ->      False
0.01       ->       True
1          ->       True
1.2        ->       True
.01        ->       True
-0.1       ->       True
-1         ->       True
1.2.3      ->      False
-01.2      ->      False
01         ->      False
-01        ->      False
001.1      ->      False
00.1       ->      False

CodePudding user response：

In your example, you can use this regex:

^-?(((?!0)[0-9] |0)(\.[0-9] )?|\.[0-9] )$

I'm not sure about 2. if that is a number. If it is, you can use:

^-?(((?!0)[0-9] |0)(\.([0-9] )?)?|\.[0-9] )$

CodePudding user response：

Extend the tests with anything applicable and use or adapt this simple re:

In [1]: tests_map = {True: ['0.01', '1', '1.2', '.01', '01'],
   ...: False: ['1.2.3']}

In [2]: pattern = r'^\d*\.{0,1}\d*$'

In [3]: import re

In [4]: for result, tests in tests_map.items():
   ...:     for test in tests:
   ...:         if not (bool(re.match(pattern, test)) == result):
   ...:             print(f"{test} was not {result}")
   ...:

You can 'read' the pattern as "zero or more decimal characters followed by zero or one decimal points followed by zero or more decimal characters".