MySQL REGEXP repeating characters

How can I find strings with individual repeating characters (for example 3 times)?

This works but I don't want rewrite every alpha characters (a,b,c,d,e...):

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... REGEXP '(a){3}|(b){3}|(c){3}|(d){3}|(e){3}|(f){3}...' ...

Thank you very much

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r = '(\w)\1{2}`

finds any alphanumeric character that is repeated (at least) three times, matching the first three.

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How about repeating a capture group?

r = '([abc])\1\1'

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'(abc){3}'

Example:

> echo 'bbbabcabcabcaaabbbccc' | egrep -o '(abc){3}'
abcabcabc