Extracting expression

Question

I have a expression and I want to extract it in python 2.6. Here is the example:

[a]+[c]*0.6/[b]-([a]-[f]*0.9)

this going to:

(
  '[a]',
  '+',
  '[c]',
  '*',
  '0.6',
  '/',
  '[b]',
  '-',
  '(',
  '[a]',
  '-',
  '[f]',
  '*',
  '0.9',
  ')',
)

I need it a list. Please give me a hand. Thanks.

I new to python. I have a backspace button and when it clicked I need remove last element of list /I'm going to use it via stack./. I think it's very easy way to do it. — Zeck, Jul 27 '11 at 09:42
So I need a list. How to extract an expression? Expression is a string. — Zeck, Jul 27 '11 at 09:45
http://stackoverflow.com/questions/3582403/safe-expression-parser-in-python — warvariuc, Jul 27 '11 at 09:45

Susam Pal · Accepted Answer · 2011-07-27T09:59:22.630

1

>>> import re
>>> expr = '[a]+[c]*0.6/[b]-([a]-[f]*0.9)'
>>> re.findall('(?:\[.*?\])|(?:\d+\.*\d*)|.', expr)
['[a]', '+', '[c]', '*', '0.6', '/', '[b]', '-', '(', '[a]', '-', '[f]', '*', '0.9', ')']

edited Jul 27 '11 at 09:59

answered Jul 27 '11 at 09:52

Susam Pal

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Zach Kelling · Answer 2 · 2011-07-27T10:02:52.057

One approach would be to create a list of regular expressions to match each token, something like:

import re
tokens = [r'\[.?\]', r'\(', r'\)', r'\+', r'\*', r'\-', r'/', r'\d+?.\d+', r'\d+']
regex = re.compile('|'.join(tokens))

Then you could use findall on your expression to return a list of matches:

>>> regex.findall('[a]+[c]*0.6/[b]-([a]-[f]*0.9)')
<<< 
['[a]',
 '+',
 '[c]',
 '*',
 '0.6',
 '/',
 '[b]',
 '-',
 '(',
 '[a]',
 '-',
 '[f]',
 '*',
 '0.9',
 ')']

Extracting expression

2 Answers2