Finding repeating operands using regex - Python

Question

I'm trying to find through a file expressions such as A*B.

A and B could be anything from [A-Z] [a-z] [0-9] and may include < > ( ) [ ] _ . etc. but not commas, semicolon, whitespace, newline or any other arithmetic operator (+ - \ *). These are the 8 delimiters. Also there can be spaces between A and * and B. Also the number of opening brackets need to be the same as closing brackets in A and B.

I unsuccessfully tried something like this (not taking into account operators inside A and B):

import re
fp = open("test", "r")
for line in fp:
    p = re.compile("( |,|;)(.*)[*](.*)( |,|;|\n)")
    m = p.match(line)
        if m:
            print 'Match found ',m.group()
        else:
            print 'No match'

Example 1:

(A1 * B1.list(), C * D * E) should give 3 matches:

A1 * B1.list()
C * D
D * E

An extension to the problem statement could be that, commas, semicolon, whitespace, newline or any other arithmetic operator (+ - \ *) are allowed in A and B if inside backets:

Example 2:

(A * B.max(C * D, E)) should give 2 matches:

A * B.max(C * D, E)
C * D

I'm new to regular expressions and curious to find a solution to this.

Use search ......... match tries to match from the begining. — Avinash Raj
– Avinash Raj, Commented Aug 24, 2015 at 13:31
You probably want to search for one or more non-separator chatacters, followed by one or more separators, followed by some non-separators again. Check out the ^. — JimmyB
– JimmyB, Commented Aug 24, 2015 at 13:35
Regular expressions is not a good tool for this particular task. Consider creating a simple parser — Konstantin
– Konstantin, Commented Aug 24, 2015 at 13:58

Community · Accepted Answer · 2017-05-23 12:22:04Z

1

Regular expressions have limits. The border between regular expressions and text parsing can be tight. IMO, using a parser is a more robust solution in your case.

The examples in the question suggest recursive patterns. A parser is again superior than a regex flavor in this area.

Have a look to this proposed solution: Equation parsing in Python.

edited May 23, 2017 at 12:22

CommunityBot

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answered Aug 25, 2015 at 9:45

Stephan

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