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

Thanks, this was my solution to the issue, `^0*1?0*1?0*1?0*1?0*1?0*$`, https://regex101.com/r/XinvHD/1, but I prefer @bunji answer. — Ibrahim, Mar 25 '18 at 00:45

score 8 · Accepted Answer · edited Mar 25 '18 at 00:40

8

^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

edited Mar 25 '18 at 00:40

Ronan Boiteau

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answered Mar 25 '18 at 00:37

bunji

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Thanks for your answer. However, `^0*(10*){0,5}$` also works. So I want to ask what are the functions of `?` and `:` there? – billyhalim25 Mar 25 '18 at 15:33
1

The `?:` makes a group [non-capturing](https://stackoverflow.com/questions/3512471/what-is-a-non-capturing-group-what-does-do). From your description it didn't seem like you needed to capture any groups so I made it non-capturing out of habit since there is a [slight cost](https://stackoverflow.com/questions/41444807/why-is-regex-search-slower-with-capturing-groups-in-python) to using capturing groups with most regex engines. You are right though, it works either way. – bunji Mar 25 '18 at 17:54

score 0 · Answer 2 · answered Mar 25 '18 at 09:21

If you engine supports lookaheads, and there can not be more than 8 times a 1 or 0 and the 1 should not occur more than 5 times, you might use:

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

Explanation

^ Begin of the string
(?= Positive lookahead that asserts that what is on the right side
- [01]{8}$ Match 8 times a 0 or 1 until the end of the string
) Close lookahead
(?! Negative lookahead that asserts that what is on the right side
- (?:0*1){6,} The pattern zero or more times a 0 followed by a 1 0*1 is not repeated 6 or more times (so 0 till 5 times is valid)
) Close negative lookahead
[01]+$ Match 0 or 1 one or more times
$ The end of the string

How do I limit a character appearance in a regex?

2 Answers2