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.

Is it possible for there to be more than one ${} in a string like ${home} ${path}? — Explosion Pills
– Explosion Pills, Commented Feb 8, 2013 at 5:26

Raymond Hettinger · Accepted Answer · 2013-02-08 06:03:19Z

1

I think you need a regular function here instead of a regular expression. The problem is that pure regular expressions can't match arbitrarily nested recursive patterns. So, they aren't much use for track matching openers and closers.

Instead, write a function to make a pass over the input string tracking the openers in a LIFO stack (just append to a regular python list) and then match closers as you find them (popping off the most recent corresponding opener):

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')

edited Feb 8, 2013 at 6:03

answered Feb 8, 2013 at 5:30

Raymond Hettinger

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1 Comment

Qtax Over a year ago

Regex can count just fine, to a finite amount. ;-) If talking about formal regular expressions that is. Regex libs like .NET can really count (you can add and subtract with balancing groups).

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