Regular expression replace function includes too much text

Question

I'm a python newbie. My script (below) contains a function named "fn_regex_raw_date_string" that is intended to convert a "raw" date string like this: Mon, Oct 31, 2011 at 8:15 PM into a date string like this: _2011-Oct-31_PM_8-15_

Question No. 1: When the "raw" date string contains extraneous characters eg (xxxxxMon, Oct 31, 2011 at 8:15 PMyyyyyy), how should I modify my regular expression routine to exclude the extraneous characters?

  I was tempted to remove my comments from the script below to make it 
     simpler to read, but I thought it might be more helpful for me to leave 
     them in the script.

Question No. 2: I suspect that I should code another function that will replace the "Oct" in "2011-Oct-31_PM_8-15_ " with "11". But I can't help wondering if there is some way to include that functionality in my fn_regex_raw_date_string function.

Any help would be much appreciated.

Thank you, Marceepoo

import sys
import re, pdb
#pdb.set_trace()

def fn_get_datestring_sysarg():
    this_scriptz_FULLName = sys.argv[0]
    try:
        date_string_raw = sys.argv[1]
    #except Exception, e:
    except Exception:
        date_string_raw_error = this_scriptz_FULLName + ':  sys.argv[1] error:  No command line argument supplied'
        print date_string_raw_error
    #returnval = this_scriptz_FULLName + '\n' + date_string_raw
    returnval = date_string_raw
    return returnval

def fn_regex_raw_date_string(date_string_raw):
    # Do re replacements
    # p:\Data\VB\Python_MarcsPrgs\Python_ItWorks\FixCodeFromLegislaturezCalifCode_MikezCode.py
    # see also (fnmatch) p:\Data\VB\Python_MarcsPrgs\Python_ItWorks\bookmarkPDFs.aab.py

    #srchstring = r"(.?+)(Sun|Mon|Tue|Wed|Thu|Fri|Sat)(, )(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)( )([\d]{1,2})(, )([\d]{4})( at )([\d]{1,2})(\:)([\d]{1,2})( )(A|P)(M)(.?+)"
    srchstring = r"(Sun|Mon|Tue|Wed|Thu|Fri|Sat)(, )(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)( )([\d]{1,2})(, )([\d]{4})( at )([\d]{1,2})(\:)([\d]{1,2})( )(A|P)(M)"

    srchstring = re.compile(srchstring)    
    replacement = r"_\7-\3-\5_\13M_\9-\11_"
    #replacement = r"_\8-\4-\6_\14M_\10-\12_"    
    regex_raw_date_string = srchstring.sub(replacement, date_string_raw)

    return regex_raw_date_string

    # Mon, Oct 31, 2011 at 8:15 PM  
if __name__ == '__main__':
    try:
        this_scriptz_FULLName = sys.argv[0]
        date_string_raw = fn_get_datestring_sysarg()
        date_string_mbh = fn_regex_raw_date_string(date_string_raw)
        print date_string_mbh
    except:
        print 'error occurred - fn_get_datestring_sysarg()'

Answer 1

This code uses a regular expression that replaces everything at the start of a string before an abbreviated weekday is matched, and then everything to the end of the string after matching either AM or PM.

Then it calls datetime.strptime(date_str, date_format) which does the hard work of parsing and gives us a datetime instance:

from datetime import datetime

import calendar
import re

# -------------------------------------

# _months = "|".join(calendar.month_abbr[1:])
_weekdays = "|".join(calendar.day_abbr)

_clean_regex = re.compile(r"""
    ^
    .*?
    (?=""" + _weekdays + """)
    |
    (?<=AM|PM)
    .*?
    $
""", re.X)

# -------------------------------------

def parseRawDateString(raw_date_str):
    try:
        date_str = _clean_regex.sub("", raw_date_str)
        return datetime.strptime(date_str, "%a, %b %d, %Y at %I:%M %p")

    except ValueError as ex:
        print("Error parsing date from '{}'!".format(raw_date_str))
        raise ex

# -------------------------------------

if __name__ == "__main__":
    from sys import argv

    s = argv[1] if len(argv) > 1 else "xxxxxMon, Oct 31, 2011 at 8:15 PMyyyyyy"

    print("Raw date:       '{}'".format(s))
    d = parseRawDateString(s)
    print("datetime object:")
    print(d)
    print("Formatted date: '{}'".format(d.strftime("%A, %d %B %Y @ %I:%M %p")))

Answer 2

You probably want to use python's standard datetime stuff:

http://docs.python.org/library/time.html#time.strptime

http://mail.python.org/pipermail/tutor/2006-March/045729.html