CODEWITHSUNDEEP
pythonregex
David Sousa
David Sousa

Reputation: 383

A simple regexp in python

My program is a simple calculator, so I need to parse te expression which the user types, to get the input more user-friendly. I know I can do it with regular expressions, but I'm not familar enough about this.

So I need transform a input like this:

import re
input_user = "23.40*1200*(12.00-0.01)*MM(H2O)/(8.314 *func(2*x+273.15,x))"
re.some_stuff( ,input_user) # ????

in this:

"23.40*1200*(12.00-0.01)*MM('H2O')/(8.314 *func('2*x+273.15',x))"

just adding these simple quotes inside the parentheses. How can I do that?

UPDATE:

To be more clear, I want add simple quotes after every sequence of characters "MM(" and before the ")" which comes after it, and after every sequence of characters "func(" and before the "," which comes after it.

Upvotes: 0

Views: 98

Answers (1)

cge
cge

Reputation: 9888

This is the sort of thing where regexes can work, but they can potentially result in major problems unless you consider exactly what your input will be like. For example, can whatever is inside MM(...) contain parentheses of its own? Can the first expression in func( contain a comma? If the answers to both questions is no, then the following could work:

 input_user2 = re.sub(r'MM\(([^\)]*)\)', r"MM('\1')", input_user)
 output = re.sub(r'func\(([^,]*),', r"func('\1',", input_user)

However, this will not work if the answer to either question is yes, and even without that could cause problems depending upon what sort of inputs you expect to receive. Essentially, the first re.sub here looks for MM( ('MM('), followed by any number (including 0) of characters that aren't a close-parenthesis ('([^)]*)') that are then stored as a group (caused by the extra parentheses), and then a close-parenthesis. It replaces that section with the string in the second argument, where \1 is replaced by the first and only group from the pattern. The second re.sub works similarly, looking for any number of characters that aren't a comma.

If the answer to either question is yes, then regexps aren't appropriate for the parsing, as your language would not be regular. The answer to this question, while discussing a different application, may give more insight into that matter.

Upvotes: 2

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