Searching words with ANY repeating characters

Question

Looking for a perl one-liner what will find all words with the next pattern:

X(not_X_chrs)X(not_X_chrs)X    e.g. cyclic

For one character, it is easy, e.g. for 'a'

perl -nle 'print if /^a[^a]+a[^a]+a$/' < /usr/share/dict/web2

but I want search for ANY character, so, looking for one regex for finding all words like:

azalea   #repeating a
baobab   #repeating b
cyclic   #c

and so on..

tried this:

perl -nle 'print if m/^([a-z])[^$1]+$1[^$1]+$1$/i' </usr/share/dict/web2

but not works.

Answer 1

(?:(?!STRING).)

is to

(?:STRING)

as

[^CHAR]

is to

CHAR

so you could use

/
   ^
   (\pL)
   (?:
      (?:(?!\1).)+
      \1
   ){2}
   \z
/sx

Answer 2

This is the best regex I could come up with:

^([a-z])((?:(?!\1).)+\1){2}$

Tested on RegexPal.