Regex: Remove all match plus one char before all

[buran]

I don't know how you time it but here is what I get:

import re
import timeit

def alfalfa(input_str=None, n=1000):
   if not input_str:
       string = 'it |BS||BS||BS|this is one|BS||BS||BS|an example'*n
   else:
       string = input_str
   while re.search("\|BS\|", string):
       array = list(string)
       for m in re.finditer("\|BS\|", string):
           del array[m.start():m.end()]
           if m.start()-1 >= 0:
               del array[m.start()-1]
           string = ''.join(array)
           break
   return string

def buran1(input_str = None, n=1000):
   ptrn = re.compile(r'[\w ]?\|BS\|')
   if not input_str:
       string = 'it |BS||BS||BS|this is one|BS||BS||BS|an example'*n
   else:
       string = input_str
   while True:
       after_sub = ptrn.sub('', string, count=1)
       if string == after_sub:
           break
       else:
           string = after_sub
   return string
   
def buran2(input_str=None, n=1000):
   ptrn = re.compile(r'(\|BS\|)+')
   if not input_str:
       string = 'it |BS||BS||BS|this is one|BS||BS||BS|an example'*n
   else:
       string = input_str
   while True:
       match = re.search(ptrn, string)
       if match:
           num_chars = min(match.start(), int(len(match.group())/4))
           sub_pattern = re.compile(r'[\w ]{{{}}}(\|BS\|)+'.format(num_chars))
           string = sub_pattern.sub('', string, count=1)
       else:
           break
   return string
   
def noBS(s=None, n=1000):
   if not s:
       s = 'it |BS||BS||BS|this is one|BS||BS||BS|an example'*n
   else:
       s = s*n
   pattern=re.compile(r'.\|BS\|((\|BS\|)*)')
   previous=''
   while s!=previous:
       previous=s
       s=re.sub(pattern,r'\1',s) 
   return s

if __name__ == '__main__':
   print 'repeat 1000, short string:\n'
   print 'alfalfa --> {}'.format(timeit.timeit("alfalfa(n=1)", number=1000, setup="from __main__ import alfalfa"))
   print 'buran1 --> {}'.format(timeit.timeit("buran1(n=1)", number=1000, setup="from __main__ import buran1"))
   print 'buran2 --> {}'.format(timeit.timeit("buran2(n=1)", number=1000, setup="from __main__ import buran2"))
   print 'ofnut --> {}'.format(timeit.timeit("noBS(n=1)", number=1000, setup="from __main__ import noBS"))
   print '\nrepeat 1, long string\n'
   print 'alfalfa --> {}'.format(timeit.timeit("alfalfa()", number=1, setup="from __main__ import alfalfa"))
   print 'buran1 --> {}'.format(timeit.timeit("buran1()", number=1, setup="from __main__ import buran1"))
   print 'buran2 --> {}'.format(timeit.timeit("buran2()", number=1, setup="from __main__ import buran2"))
   print 'ofnut --> {}'.format(timeit.timeit("noBS()", number=1, setup="from __main__ import noBS"))

and the result of two consecutive runs:

Output:
repeat 1000, short string:

alfalfa --> 0.0432239843385
buran1 --> 0.0112259009714
buran2 --> 0.0158689890339
ofnut --> 0.0273017555023

repeat 1, long string

alfalfa --> 3.50733362241
buran1 --> 1.34837528801
buran2 --> 1.86298544437
ofnut --> 0.0084199068111

repeat 1000, short string:

alfalfa --> 0.0284217156815
buran1 --> 0.00996738901746
buran2 --> 0.0157894500521
ofnut --> 0.0273982927342

repeat 1, long string

alfalfa --> 3.52313333556
buran1 --> 1.35965603239
buran2 --> 1.82195551718
ofnut --> 0.00834742370672