python - Regex needed for variable expansion in python

Question

I am having a variable having a value something like this

var = '${home_${path}}'

I want a regex which detects ${ any content } recursively. That is, first I should match ${home_${path}} and then ${path}. I need this to evaluate the variable content by replacing ${..} with its actual content.

Answer 1

我认为你需要一个正则函数而不是正则表达式。问题是纯正则表达式无法匹配任意嵌套的递归模式。因此，它们对于赛道匹配的开启者和关闭者没有多大用处。

相反，编写一个函数来传递跟踪 LIFO 堆栈中的开启者的输入字符串（只需附加到常规 python 列表），然后在找到它们时匹配关闭器（弹出最近的相应开启者）：

import re

braces = re.compile(r'(\${)|(})')

def substitute(s, **env):
    openers = []
    pos = 0
    while 1:
        mo = braces.search(s, pos)
        if mo is None:
            break
        opener, closer = mo.groups()
        if opener:
            openers.append(mo.start())
            pos = mo.end() + 1
            continue
        start = openers.pop()
        end = mo.end()
        body = s[start+2 : end-1]
        s = s[:start] + env[body] + s[end:]
    return s


print substitute('Return to ${home_${path}} for further ${action}',
                 home_world='earth',
                 home_state='kansas',
                 path='state',
                 action='orders')