在 Python 中查找最长匹配前缀

Question

I have to check if a line starts with options as ["<", "<=", ">", ">=", "=="]. I have tried:

line = "<=5"
my_option = ["<", "<=", ">", ">=", "=="]
for op in my_option:
    if line.startswith(op):
       print(op)

Doing so, I get both "<" and "<=" as output but I am interested in only output "<=".

Answer 1

Re order the list, so that items that are prefixes of others are later, and break after first match:

line = "<=5"
my_option = ["<=", ">=", "==", ">", "<"]
for op in my_option:
    if line.startswith(op):
       print(op)
       break

In that manner, the line first matched against <= and we get out the loop.

Answer 2

We can get the longest matching prefix using regex. Make my_options a set and search prefix in it like the following:

import re

my_option = {"<", "<=", ">", ">=", "=="}
line = "<=5"

prefix = re.split(r'(-?\d*\.?\d+)', line)[0]

if prefix in my_option:
    print(prefix)

The above code would split line with a regex that matches float or integer numbers with an optional minus sign. To match a negative or positive number having exponent, replace the above regex with:

r'([+-]?\d*\.?\d+(?:[eE][-+]?\d+)?)'