Question

I would like to use a regular expression to match all words with more that one character, as opposed to words entirely made of the same char.

This should not match: ttttt, rrrrr, ggggggggggggg

This should match: rttttttt, word, wwwwwwwwwu

Answer 1

The following expression will do the trick.

^(?<FIRST>[a-zA-Z])[a-zA-Z]*?(?!\k<FIRST>)[a-zA-Z]+$

• capture the first character into the group FIRST
• capture some more characters (lazily to avoid backtracking)
• ensure that that the next character is different from FIRST using a negative lookahead assertion
• capture all (at least one due to the assertion) remaining characters

Note that is sufficient to look for a character that is different from the first one, because if no character is different from the first one, all characters are equal.

You can shorten the expression to the following.

^(\w)\w*?(?!\1)\w+$

This will match some more characters other than [a-zA-Z].