Question

I don’t know if that’s the right word for it, but I am trying to come up with some regex’s that can extract coefficients and exponents from a mathematical expression. The expression will come in the form ‘axB+cxD+exF’ where the lower case letters are the coefficients and the uppercase letters are the exponents. I have a regex that can match to both of them, but I’m wondering if I can use 2 regexs, one to match the coefficients and one for the exponents. Is there a way to match a number with a letter on one side of it without matching the letter? EG, in ‘3×3+6×2+2×1+8×0’ I need to get
[‘3’, ‘6’, ‘2’, ‘8’]
and
[‘3’, ‘2’, ‘1’, ‘0’]

Answer 1

You can use positive look-ahead to match something that is followed by something else. To match the coefficients, you can use:

>>> s = '3x3+6x2+2x1+8x0'
>>> re.findall(r'\d+(?=x)', s)
['3', '6', '2', '8']

From the documentation of the re module:

(?=…)
Matches if … matches next, but doesn’t consume any of the string.
This is called a lookahead assertion.
For example, Isaac (?=Asimov) will
match ‘Isaac ‘ only if it’s followed
by ‘Asimov’.

For the exponents, you can use positive look-behind instead:

>>> s = '3x3+6x2+2x1+8x0'
>>> re.findall(r'(?<=x)\d+', s)
['3', '2', '1', '0']

Again, from the docs:

(?<=…) Matches if the current position in the string is preceded by a match for
… that ends at the current position.
This is called a positive lookbehind
assertion. (?<=abc)def will find a
match in abcdef, since the lookbehind
will back up 3 characters and check if
the contained pattern matches.