Regex, how to remove all non-alphanumeric except colon in a 12/24 hour timestamp?

Question

I have a string like:

Today, 3:30pm - Group Meeting to discuss "big idea"

How do you construct a regex such that after parsing it would return:

Today 3:30pm Group Meeting to discuss big idea

I would like it to remove all non-alphanumeric characters except for those that appear in a 12 or 24 hour time stamp.

Answer 1

# this: D:DD, DD:DDam/pm 12/24 hr
re = r':(?=..(?<!\d:\d\d))|[^a-zA-Z0-9 ](?<!:)'

A colon must be preceded by at least one digit and followed by at least two digits: then it's a time. All other colons will be considered textual colons.

How it works

:              // match a colon
(?=..          // match but not capture two chars
  (?<!         // start a negative look-behind group (if it matches, the whole fails)
    \d:\d\d    // time stamp
  )            // end neg. look behind
)              // end non-capture two chars
|              // or
[^a-zA-Z0-9 ]  // match anything not digits or letters
(?<!:)         // that isn't a colon

Then when applied to this silly text:

Today, 3:30pm - Group 1,2,3 Meeting to di4sc::uss3: 2:3:4 "big idea" on 03:33pm or 16:47 is also good

...changes it into:

Today, 3:30pm  Group 123 Meeting to di4scuss3 234 big idea on 03:33pm or 16:47 is also good

Answer 2

Python.

import string
punct=string.punctuation
s='Today, 3:30pm - Group Meeting:am to discuss "big idea" by our madam'
for item in s.split():
    try:
        t=time.strptime(item,"%H:%M%p")
    except:
        item=''.join([ i for i in item if i not in punct])
    else:
        item=item
    print item,

output

$ ./python.py
Today 3:30pm  Group Meetingam to discuss big idea by our madam

# change to s='Today, 15:30pm - Group 1,2,3 Meeting to di4sc::uss3: 2:3:4 "big idea" on 03:33pm or 16:47 is also good'

$ ./python.py
Today 15:30pm  Group 123 Meeting to di4scuss3 234 big idea on 03:33pm or 1647 is also good

NB: Method should be improved to check for valid time only when necessary(by imposing conditions) , but i will leave it as that for now.