Plain english description for this regular expression

Question

What would be a a description in words of the general string that this RE accepts?

(01(10)*11)*

I first thought that it was just any string beginning and ending with 01 and 11 respectively, with any number of alternating 1s and 0s inbetween, but the fact that the entire regex is contained in ()* voids this description.

read the explanation box on regex101.com for your regex. It's the most complete explanation you'll get — baao, Mar 05 '17 at 22:08

Casimir et Hippolyte · Answer 1 · 2017-03-05T22:47:08.950

2

(01(10)*11)*

zero or more repeats of a group composed by:

01
zero or more repeats of a group composed by: 10
11

This pattern can match for example:

the empty string
0111 => 01##11
011011 => 01#10#11
01101011 => 01#1010#11
0110101011 => 01#101010#11
011011011011 => 01#10#11|01#10#11
011011011011011011 => 01#10#11|01#10#11|01#10#11
011101101101101011 => 01##11|01#10#11|01#1010#11
011010110110110111 => 01#1010#11|01#10#11|01##11

edited Mar 05 '17 at 22:47

answered Mar 05 '17 at 22:16

Casimir et Hippolyte

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Your example setup is better than mine. :) – Regular Jo Mar 05 '17 at 22:30
@cfqueryparam: thanks, but not sure of that (*vox populi, vox dei*) – Casimir et Hippolyte Mar 05 '17 at 22:31

Regular Jo · Answer 2 · 2017-03-06T06:32:29.777

(        # opens capture group 1
  01     # matches 01, literally
  (      # opens capture group 2
    10   # matches 10, literally
  )      # closes  capture group 2
  *      # Repeats previous group zero or more times
  11     # matches 11, literally
)      
*        # Repeats previous group zero or more times

Capturing group 2 is only reached when all the other components of capturing group 1 are met. If it can't find 11 after any number of 10s, it backtracks out and keeps looking.

This would match 01 followed by any number of 10, including 0/none, followed by `11.

This would match

             - nothing, zero repeats of the outer group
             - regexes that can match an empty string are useless for
             - validation, especially without assertions.
0111         - zero occurrencess of 10
011011       - one occurrence of 10.
01101011     - two occurrences of 10
011101101011 - one occurrence of CG1 where CG2 had no iterations,
               - and one where it had 2
             - this can and will continue to an infinite number of matches

Please read up on Quantifiers and honestly, you can do a great deal of learning experimenting on regex101. Here's your regex setup there. All the purple dots are because your expression can match empty strings.

Plain english description for this regular expression

2 Answers2