I'm trying to diff two strings to determine whether or not they solely vary in one numerical subset of the string structure; for example,

```
varies_in_single_number_field('foo7bar', 'foo123bar')
# Returns True, because 7 != 123, and there's only one varying
# number region between the two strings.

```

In Python I can use the `difflib` to accomplish this:

```
import difflib, doctest

def varies_in_single_number_field(str1, str2):
    """
    A typical use case is as follows:
        >>> varies_in_single_number_field('foo7bar00', 'foo123bar00')
        True

    Numerical variation in two dimensions is no good:
        >>> varies_in_single_number_field('foo7bar00', 'foo123bar01')
        False

    Varying in a nonexistent field is okay:
        >>> varies_in_single_number_field('foobar00', 'foo123bar00')
        True

    Identical strings don't *vary* in any number field:
        >>> varies_in_single_number_field('foobar00', 'foobar00')
        False
    """
    in_differing_substring = False
    passed_differing_substring = False # There should be only one.
    differ = difflib.Differ()
    for letter_diff in differ.compare(str1, str2):
        letter = letter_diff[2:]
        if letter_diff.startswith(('-', '+')):
            if passed_differing_substring: # Already saw a varying field.
                return False
            in_differing_substring = True
            if not letter.isdigit(): return False # Non-digit diff character.
        elif in_differing_substring: # Diff character not found - end of diff.
            in_differing_substring = False
            passed_differing_substring = True
    return passed_differing_substring # No variation if no diff was passed.

if __name__ == '__main__': doctest.testmod()

```

But I have no idea how to find something like `difflib` for C++. Alternative approaches welcome. :)