How to write a regex which starts with a given number?

Regex is overkill:

line = '5p.m. its a lovely time for tea!'
num = raw_input()  # integer input
if line.startswith(num):
    print 'OK'
else:
    print 'It\'s not OK'

If you need to use regex, you can do this:

if re.search('^%s' %num, line):  # checks if there is a match
    print 'OK'
else:
    print 'It\'s not OK'

The reason this works is because if there is a match, re.search will return an _sre.SRE_Match object will evaluate to True in a boolean context. If there is no match, it will return None will evaluates to False.

I don't know about c++ and python coding but regex that you want is just

^(\d).*

you can use http://regexpal.com/ to check result

PS. Sorry i try to post image but they don't allow so click the link to see https://i.sstatic.net/qwBjJ.png

import re
number = 5
line = "5p.m. its a lovely time for tea!"
s = compile("^%d" % num)
m = s.search(line)

if m:
  print "time for tea"

import re

line = "5p.m. its a lovely time for tea!"

match_obj = re.search('^([0-9]+).*', line)

num = raw_input()

if num == match_obj.groups()[0]:

    print "OK"

else:

    print r"It's not ok"

I would echo the response above: regex is overkill for this problem. Use startswith as pointed out above.