Regular expression - number without same digit beside

Question

I'm looking for a regex, which fits to every number, that is made from 0,1,2 and don't have the same digits beside - 02021 fits, 0122 doesn't fit. How it can be written?

Answer 1

You can start with a digit between zero and two. Capture that in a group and check that the following is not the captured value. Capture that in a non capturing group and repeat that.

^(?:([0-2])(?!\\1))*$

Explanation

• From the beginning of the string ^
• A non capturing group (?:
• A capturing group (
• A character range from 0 till 2 [0-2]
• Close capturing group )
• A negative lookahead (?!
• Assert that what is following is not equal to the first capturing group
• Close the negative lookahead )
• Close the non capturing group )
• Repeat the non capturing group zero or more times *
• The end of the string $

Answer 2

You can use the following regex:

^(([012])(?!\2))+$

tested here: https://regex101.com/r/6vevDl/1