a regular expression generator for number ranges

Question

I checked on the stackExchange description, and algorithm questions are one of the allowed topics. So here goes.

Given an input of a range, where begin and ending number

Answer 1

Here is a recursive solution in python, which works for an arbitrary range of positive numbers. The idea is to divide the range into three sub-ranges:

• from start to the next multiple of 10 (if start is not already a multiple of 10)
• from the last multiple of 10 to end (if end is not already a multiple of 10)
• the range between these two multiples of 10 can be handled recursivle by taking off the last digit and adding the regular expression [0-9] to all generated regular expressions afterwards

The code below even optimizes ranges of single values like [1-1] to 1. The function to call is genRangeRegex (start is inclusive, end is exclusive):

def regexRangeDigits(start,stop):
  if start == stop:
    return str(start)
  return '[%d-%d]' % (start,stop)

# generate list of regular expressions for the number range [start,end[
def genRangeRegex(start, end):
  if start <= 0:
    raise ValueError('only ranges of positive numbers supported')

  print 'getting regex list for range [%d,%d[' % (start,end)
  if start >= end:
    return []

  digitsStart = str(start)
  digitsEnd   = str(end)
  lastDigitStart = start%10

  if start//10 == (end-1)//10: # integer division
    lastDigitStop = (end-1)%10
    regexAll = digitsStart[:-1] + regexRangeDigits(lastDigitStart,lastDigitStop)
    print '  regexAll   = %s' % regexAll
    return [regexAll]

  regexListStart = [] # at most one regular expression for going up to first multiple of 10
  if lastDigitStart != 0:
    regexStart = digitsStart[:-1] + regexRangeDigits(lastDigitStart,9)
    print '  regexStart = %s' % regexStart
    regexListStart.append(regexStart)

  regexListEnd = [] # at most one regular expression for going up from last multiple of 10
  lastDigitEnd = end%10
  if lastDigitEnd != 0:
    regexEnd = digitsEnd[:-1] + regexRangeDigits(0,lastDigitEnd-1)
    print '  regexEnd   = %s' % regexEnd
    regexListEnd.append(regexEnd)

  regexListMidTrunc = genRangeRegex((start+9)//10, end//10)
  regexListMid = [r+'[0-9]' for r in regexListMidTrunc]

  return regexListStart + regexListMid + regexListEnd

And here an example output how the function works:

>>> genRangeRegex(12,231)
getting regex list for range [12,231[
  regexStart = 1[2-9]
  regexEnd   = 230
getting regex list for range [2,23[
  regexStart = [2-9]
  regexEnd   = 2[0-2]
getting regex list for range [1,2[
  regexAll   = 1
['1[2-9]', '[2-9][0-9]', '1[0-9][0-9]', '2[0-2][0-9]', '230']