Parsing strings using regular expressions received by a GSM modem connected to a laptop via a serial port in python

Question

Here is the function I am using to poll for new messages received by a GSM modem connected to my laptop via a serial port. I have created a serial object 'phone' which I am using to read and write data.

'phone' reads the data into a list x.

The response I get when I get a new message is of the format:

+CMTI: "SM",0

Here 0 is an index, indicating the index position at which the message is stored on the SIM. The following code is used to poll the list x which contains the data the I am reading and writing from the serial port. I am using regular expressions to parse x which has been converted to a string myRegexableString to see whether I have got the notification or not.

def poll(x):
    myRegexableString = "".join(x)
    print "Printing myRegexableString"
    pprint(myRegexableString)
    regex = re.compile(r'\+CMTI: "SM",(\d+)')
    lst = []
    for l in myRegexableString:
        for m in [regex.search(l)]:
            if myRegexableString:
                lst.append(m)
                print "You have received a new message!"

I want to use the above code for a AT command that checks for network registration. I will need to poll for new messages while I am executing other commands too, or any command for that matter but for the sake of this question I am just going to take the example of network registration. Here is how it would normally run without using the polling function.

AT+CREG? #command

+CREG: 0,1 #response

OK #status

Now here's the code for that.

def register():
   print "Checking for network registration"
   phone.write(b'AT+CREG?\r')
   sleep()
   x=phone.read(50)
   sleep()
   print x

Now suppose I include polling here the code would change to

def register():
       print "Checking for network registeration"
       phone.write(b'AT+CREG?\r')
       sleep()
       x=phone.read(50)
       poll(x)
       sleep()
       print x

The output I am getting from the changed code is

Checking for network registeration
Printing myRegexableString
'AT+CREG?\r\r\n+CREG: 0,1\r\n\r\nOK\r\n'
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
You have received a new message!
AT+CREG?

+CREG: 0,1

OK

Why is it printing "You have received a new message!" so many times?

Especially when I have not received a new message!

Please help.

Martijn Pieters Martijn Pieters · Accepted Answer · 2013-03-29T11:21:10

All you need to do is use regex.findall(myRegexableString). This returns a list of matched groups, so numbers.

regex = re.compile(r'\+CMTI: "SM",(\d+)')

def poll(x):
    myRegexableString = "".join(x)
    print "Printing myRegexableString"
    pprint(myRegexableString)
    lst = regex.findall(myRegexableString)

I've moved the re.compile() call out of the function, there is no need to re-compile the expression every time.

You are looping over each and every character of myRegexableString, then you loop over a list with one match, then you test if myRegexableString is True-ish (which it is if it is not empty). This means that for every character in the input you add an empty match (the expression will never match anything in a single character string).

Parsing strings using regular expressions received by a GSM modem connected to a laptop via a serial port in python

1 Answers