splitting using regex python for complicated strings

Question

I want to splitting a string such as Si(C3(COOH)2)4(H2O)7 into the following

[Si, (C3(COOH)2), 4, (H2O), 7]

That is, entire paranthesis expressions turn into an element by themselves. I've tried a number of different combinations with re.findall() to no avail. Any help is greatly appreciated.

You can't parse nested structures with regular expressions in Python, so there's no way to find out which `(` or `)` to split on. — Zero Piraeus, Nov 11 '16 at 18:44

Terry Jan Reedy · Answer 1 · 2016-11-11T22:47:05.440

You have to scan the string yourself, keeping track of the nesting depth. The significant 'events' are 'at beginning of string', 'at (', 'at )', and 'at end of string'. At each event, consider depth and reset it.

inn = 'Si(C3(COOH)2)4(H2O)7'
out = ['Si', '(C3(COOH)2)', '4', '(H2O)', '7']
res = []
beg = 0
dep = 0
for i, c in enumerate(inn):
    if c == '(':
        if dep == 0 and beg < i:
            res.append(inn[beg:i])
            beg = i
        dep += 1
    elif c == ')':
        if dep == 0:
            raise ValueError("')' without prior '('")
        elif dep == 1:
            res.append(inn[beg:i+1])
            beg = i+1
        dep -= 1
if dep == 0:
    res.append(inn[beg:i+1])
else:
    raise ValueError("'(' without following ')'")
print(res, res == out)

# prints
# ['Si', '(C3(COOH)2)', '4', '(H2O)', '7'] True

splitting using regex python for complicated strings

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