Evaluate expression in a string in python

Question

I have a string with mathematical operations, how do I evaluate the string within [] to generate the output shown below?

mystring = "[2*3-1] plus [4+2] is equal to [5+6]."

re.findall(r"\[(.*?)\]",mystring) # to find 2*3-1, 4+2, 5+6

Output: "5 plus 6 is equal to 11." # expected output.

Tim Biegeleisen · Accepted Answer · 2021-05-13 07:02:33Z

3

You may use re.sub with eval and a callback function here:

mystring = "[2*3-1] plus [4+2] is equal to [5+6]."
output = re.sub(r'\[(.*?)\]', lambda m: str(eval(m.group(1))), mystring)
print(output)  # 5 plus 6 is equal to 11.

The strategy here is to match the aritrmetic expression inside every [...], which then gets passed to a lambda function. There, we call eval() to get the result, cast to a string and generate the replacement.

edited May 13, 2021 at 7:02

answered May 13, 2021 at 6:59

Tim Biegeleisen

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4 Comments

anubhava Over a year ago

++ but OP doesn't need [ and ] in output

Tim Biegeleisen Over a year ago

@anubhava On a cell phone with bad visibility to the entire screen at once, sorry.

anubhava Over a year ago

You wrote this formatted code on a mobile, wish I could give 10 ++ to you

Tim Biegeleisen Over a year ago

Still trying to find a software shop where mobile typing skills gets you promoted :-)

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