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Asked: 2026-05-25T20:21:19+00:00

I have the below regular expression in Python, ^1?$|^(11+?)\1+$ Since there is a pipe

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I have the below regular expression in Python,

^1?$|^(11+?)\1+$

Since there is a pipe ‘|’, I will split it into 2 regex,

^1?$

For this, it should validate 1 or empty value. Am I correct?

^(11+?)\1+$

For the above regex, it would validate value of 1111. The first pair of 11 is based on (11+?) and the second pair of 11 is due to \1.

When I attempt to execute it in Python, it returns true only for 1111 but not 11 or empty value. Am I wrong somewhere?

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1 Answer

  1. Editorial Team

    Ted wrote:

    For this, it should validate 1 or empty value. Am I correct?

    Yes, that is correct.

    Ted wrote:

    When I attempt to execute it in Python, it returns true only for 1111 but not 11 or empty value. Am I wrong somewhere?

    The empty string does get matched. The following snippet:

    #!/usr/bin/env python
import re

for n in xrange(0, 51):
  ones = '1' * n
  matches = re.match(r'^1?$|^(11+?)\1+$', ones)
  if matches:
    div1 = n if matches.group(1) is None else len(matches.group(1))
    div2 = 0 if div1 is 0 else len(ones)/div1
    print "[{0:2}]:{1:2} * {2:2} = '{3}'".format(n, div1, div2, ones)

    will print:

    [ 0]: 0 *  0 = ''
[ 1]: 1 *  1 = '1'
[ 4]: 2 *  2 = '1111'
[ 6]: 2 *  3 = '111111'
[ 8]: 2 *  4 = '11111111'
[ 9]: 3 *  3 = '111111111'
[10]: 2 *  5 = '1111111111'
[12]: 2 *  6 = '111111111111'
[14]: 2 *  7 = '11111111111111'
[15]: 3 *  5 = '111111111111111'
[16]: 2 *  8 = '1111111111111111'
[18]: 2 *  9 = '111111111111111111'
[20]: 2 * 10 = '11111111111111111111'
[21]: 3 *  7 = '111111111111111111111'
[22]: 2 * 11 = '1111111111111111111111'
[24]: 2 * 12 = '111111111111111111111111'
[25]: 5 *  5 = '1111111111111111111111111'
[26]: 2 * 13 = '11111111111111111111111111'
[27]: 3 *  9 = '111111111111111111111111111'
[28]: 2 * 14 = '1111111111111111111111111111'
[30]: 2 * 15 = '111111111111111111111111111111'
[32]: 2 * 16 = '11111111111111111111111111111111'
[33]: 3 * 11 = '111111111111111111111111111111111'
[34]: 2 * 17 = '1111111111111111111111111111111111'
[35]: 5 *  7 = '11111111111111111111111111111111111'
[36]: 2 * 18 = '111111111111111111111111111111111111'
[38]: 2 * 19 = '11111111111111111111111111111111111111'
[39]: 3 * 13 = '111111111111111111111111111111111111111'
[40]: 2 * 20 = '1111111111111111111111111111111111111111'
[42]: 2 * 21 = '111111111111111111111111111111111111111111'
[44]: 2 * 22 = '11111111111111111111111111111111111111111111'
[45]: 3 * 15 = '111111111111111111111111111111111111111111111'
[46]: 2 * 23 = '1111111111111111111111111111111111111111111111'
[48]: 2 * 24 = '111111111111111111111111111111111111111111111111'
[49]: 7 *  7 = '1111111111111111111111111111111111111111111111111'
[50]: 2 * 25 = '11111111111111111111111111111111111111111111111111'

    And the input 11 is not matched because 11 is matched in group 1 ((11+?)), which should then be repeated at least once (\1+), which is not the case (it is not repeated).

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