How to use Python regular expressions for reverse Polish expression evaluation

What is reverse Polish expression?

The reverse Polish expression, also known as a postfix expression, is a method of expressing arithmetic expressions that does not require parentheses to distinguish operator precedence. Its characteristic is that the operator is behind the operand. For example, converting the infix expression "3 4 5" into a reverse Polish expression is "3 4 5 ".

What are Python regular expressions?

Python regular expressions are tools for matching and processing text data. Regular expressions can be used to search for patterns in text. Python provides the "re" module to use regular expression functionality.

How to use Python regular expressions for reverse Polish expression evaluation?

Implementing the reverse Polish expression evaluation algorithm requires following the following steps:

1. Convert the reverse Polish expression into a list so that each operator and operand can be checked in order;
2. Create an empty stack;
3. Traverse the converted list of reverse Polish expressions and perform the following operations for each element:
   a. If this is a number, push it Top of the stack;
   b. If this is an operator, pop the two elements at the top of the stack to perform the corresponding operation, and push the result to the top of the stack;
4. When the traversal is completed, the elements at the top of the stack are The evaluation result of the reverse Polish expression.

We can use Python regular expressions to easily implement the function of converting reverse Polish expressions into lists. The sample code is as follows:

import re

expression = "3 4 5 * +"
tokens = re.findall("d+|S", expression)

print(tokens) # ['3', '4', '5', '*', '+']

Then, we can follow the above steps to implement the evaluation algorithm of the reverse Polish expression. The sample code is as follows:

stack = []
for token in tokens:
    if re.match("d+", token):
        stack.append(int(token))
    else:
        operand2 = stack.pop()
        operand1 = stack.pop()
        if token == "+":
            stack.append(operand1 + operand2)
        elif token == "-":
            stack.append(operand1 - operand2)
        elif token == "*":
            stack.append(operand1 * operand2)
        elif token == "/":
            stack.append(int(operand1 / operand2))

result = stack.pop()

print(result) # 23

This code creates an empty stack, iterates through the reverse Polish expression list, checks each operator and operand, performs the corresponding operation on the stack, and finally returns the element at the top of the stack as a result.

Conclusion

Using Python regular expressions you can easily convert reverse Polish expressions into lists and perform arithmetic calculations on the stack. Python's regular expressions are very powerful and can help us achieve fast, flexible and reliable text matching and processing.

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