Using regex to select elements of a string with python

Imagine you have a list of string elements, such as you might get after reading in a text file line by line that looks as follows:

data=['month::5-->', 'day::13-->']

From the above, it looks like a value is provided between some label e.g. month:: and an end string e.g. -->. If for example you wanted to get at the month part, you could access it as follows from which point you could start messing around with splitting etc:

list(filter(lambda k: 'month::' in k, data))

Say your data looks like the following though as the file contained some unexpected gaps:

data=['address::1, Humpty Dumpty Lane,', 'Somewhere -->', 'month::5-->', 'day::13-->']

Notice how the strings that were bordering the info are different list elements now?

>>'address::1, Humpty Dumpty Lane,'

>>'Somewhere -->'

If I want to get the address, I could try the filter approach but it won’t get everything I need as really I need it to continue up to the --> which is now in a different list element:

list(filter(lambda k: 'address::' in k, data))
>> ['address::1, Humpty Dumpty Lane,']

A more efficient and tidy way to deal with this is to use regular expressions through the re package. First, concatenate everything into a string:

all_content=' '.join(data) 

Now use the following:

import re

address_info=re.findall("address::(.*?)-->", all_content)

print(address_info) # the list
>>['1, Humpty Dumpty Lane, Somewhere']
print(address_info[0]) # the string
>>1, Humpty Dumpty Lane, Somewhere

This looks for the first instance of address::, then with (.*?) gets all text up to the first instance of -->. If you just pass (.*) it will be greedy and get all text up to the last occurrence of --> which matters if you have a few of them!

For help with finding and testing regex commands, check out pythex. You can see it working with the example provided here.

Thanks John Stevenson for the help!

Written on May 13, 2020