Python Search repeated operators in string

Question

For the purpose of writing a calculator, like the python interpeter, I want to check the validity of my expressions.

I want to check a string for repeated mathematical operators, I don't want to catch anything, just to know if they exist, in which case the expression would be invalid.

4++-+4 is valid.

4*-8 is invalid

4-/7 is invalid

4/-4 is valid, mine probably fails here.

minut and plus can repeat themselves, but -* is, for example, invalid. Much like the way the python interpeter works. This is what I have, as a Regex, but any simpler solution is welcome, even not regex is great.

[*/^%\-+][*/^%] | [\-+*/^%][*/^%]

Link

Basicially, check if operators */^%-+ are either preceded by or followed by */^% (without minus and plus)

Operators are not seperated by spaces, and spaces don't exist in my text.(should I have mentioned this?) — , Nov 17 '16 at 20:03
What exactly should `^^` yield? A cat? You might want to redefine your problem :) — Jan, Nov 17 '16 at 20:03
If you capture the first character in a group, you can check to see if the next character is the same using `([*\/^@&%$\-+])(\1)` — Patrick Haugh, Nov 17 '16 at 20:03
Oh, well, it is one of my operators, I will remove those. Sorry and thanks. — , Nov 17 '16 at 20:06
So you don't even care that they are the same operator. Isn't that just `[*/^%\-+][*/^%]` — Patrick Haugh, Nov 17 '16 at 20:08
I see. I'll keep trying at regexs, but you might want to take a look at context free grammars. This looks like a problem that would be very simple for a CFG — Patrick Haugh, Nov 17 '16 at 20:18
Your examples are wrong. `4*-8` is valid (four times negative eight) — Patrick Haugh, Nov 17 '16 at 20:22
And `*+` is a grey area `4*+4` could be four times positive four — Patrick Haugh, Nov 17 '16 at 20:23
Do you need a clean solution or something mildly hacky right now? I can help in the second case ^^ — themistoklik, Nov 17 '16 at 20:27
Also [see](http://stackoverflow.com/questions/13421424/how-to-evaluate-an-infix-expression-in-just-one-scan-using-stacks) this one, if going the regex way won't do for you. — themistoklik, Nov 17 '16 at 20:30
I will look into cfg and the other thread, but a hacky solution could be nice as well — , Nov 17 '16 at 20:42

themistoklik · Answer 1 · 2016-11-17T22:46:28.860

Again a more concise solution would be either a CFG or a stack based approach for infix expressions. However something you could hack and experiment with is the following idea.

construct a product of all operators like so:

from itertools import product as p
all=list(p('*/^%-+',repeat=2))
all=map(lambda x:''.join(x),all)

invalids=[..write them by hand in here(hacky part)]
valids=filter(lambda x:x not in invalids,all)

And now you're left with all valid operations of length 2. You can scan your string with a window of 2, and when you find a pair of operators not belonging in the valids you can declare the expression invalid and move on.

Another way you could go about it is a rule-based one. Construct a dictionary with operators as keys, and for each operator the value would be a list holding all operators that can follow it.

Then your problem becomes one of checking your string at character i with the validity condition being

string[i+1] in dictionary[string[i]]

If you do find a CFG solution and its beautiful, let me know

Python Search repeated operators in string

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