Regex for matching optional parameters as groups in string

Question

I am trying to read a batch script containing an input string of a command and optional parameters like

input = 'command -a field1 -b field2 -c field3'

The option parameters are set by a user in a file and I am obtaining the string by reading the file. If the option is included I would need to extract the field.

I currently have my regex as:

expr = '^(?P<exec>[^\s]+) -m (?P<mode>[^\s]+) -d (?P<machine>[^\s]+) -p (?P<port>[^\s]+)'
m = re.match(expr, input)

When the user includes all the options in the same order, the regex matches the string and the groups are captured. Here is a sample output.

{   'exec': 'custom-script',
    'mode': 'debug',
    'machine': 'desk-123',
    'port': '7905'   }

However, if any option is not included, or if they are in different orders, the regex fails. How do I modify my regex to perform in these two cases?

Answer 1

In Python you can use the built in optparse or argparse modules to do this for you so you don't have to fidget with hardcore regular expressions which will fail eventually:-)

Answer 2

parsing command line options with regular expressions in python is very fragile - argparse will let you handle all of the arguments you want, both optional and positional, and provides abundant faculties for manipulating them. It even automatically builds a -h option for you!

For example, in your case you said you have the input supplied to you, so you could do the following:

parser = argparse.ArgumentParser(description='Process some integers.')
parser.add_argument('-m','--mode', help='The mode to run in')
parser.add_argument('-d','--machine', help='The machine of interest')
parser.add_argument('-p','--port', help='The port of interest')
args = parser.parse_args(input.split())

The output from this, args , will have the different fields as attributes, ie, args.mode , args.machine , args.port

Answer 3

Thanks to Nate, I used optparse instead of regexes and used the following code:

parser = optparse.OptionParser(description='Process some integers.')
parser.add_option('-m','--mode', help='The mode to run in')
parser.add_option('-d','--machine', help='The machine of interest')
parser.add_option('-p','--port', help='The port of interest')
(args, others) = parser.parse_args(input.split())

Now, I can access the values by args.mode, args.machine, etc.