How do I limit a character appearance in a regex?

Question

I am stuck with a problem that challenges me to create the regular expression for binary numbers (containing 0s and 1s). But the string should only match if the binary number only contains a maximum of five 1s. How do I limit a character appearance in a regex?

Examples:

• 01101101 is correct
• 01111100 is correct
• 10110011 is correct
• 01111110 is wrong
• 11111110 is wrong

Answer 1

^0*(?:10*){,5}$

Essentially this matches any combination of '1's and '0's but only allows a substring containing a single '1' character to occur five times at most.

Try it out here: https://regex101.com/r/JKV1Uk/2

Explanation:

• ^ matches the beginning of the string

• 0* matches zero or more '0's

• (?:10*){,5} matches up to 5 '1's followed by any number of zeros

• $ matches the end of the string