How to limit the appearance of a character in a regular expression?

Question

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

Examples:

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

Answer 1

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

Essentially, this matches any combination '1'and '0', but assumes that a substring containing a single character '1'will be executed a maximum of five times.

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

Explanation:

• ^ matches the beginning of a line

• 0* '0' s

• (?:10*){,5} 5 '1',

• $

Answer 2

lookaheads 8 1 0, 1 5 , :

^(?=[01]{8}$)(?!(?:0*1){6,})[01]+$

• ^
• (?= , , ,
  • [01]{8}$ 8 0 1
• )
• (?! , ,
  • (?:0*1){6,} a 0, 1 0*1, 6 ( 0 5 )
• )
• [01]+$ 0 1
• $