Question

Can regular expressions be used to perform arithmetic? Such as find all numbers in a file and multiply them by a scalar value.

Answer 1

I prepared a small script which uses re.finditer to find all the integers (you can change the regexp so that it can deal with floats or scientific notation) and then use map to return a list of scaled numbers.

import re

def scale(fact):
    """This function returns a lambda which will scale a number by a                           
    factor 'fact'"""
    return lambda val: fact * val

def find_and_scale(file, fact):
    """This function will find all the numbers (integers) in a file and                        
    return a list of all such numbers scaled by a factor 'fact'"""
    num = re.compile('(\d+)')
    scaling = scale(fact)
    f = open(file, 'r').read()
    numbers = [int(m.group(1)) for m in num.finditer(f)]
    return map(scaling, numbers)

if __name__ == "__main__":
    import sys
    if len(sys.argv) != 3:
        print "usage: %s file factor" % sys.argv[0]
        sys.exit(-1)
    numbers = find_and_scale(sys.argv[1], int(sys.argv[2]))
    for number in numbers:
        print "%d " % number

If you have a file whose numbers you want to scale by a factor fact, you call the script from the command line as python script.py file fact and it will print to STDOUT all the scaled numbers. Of course, you can do something more useful if you wanted…