Regular expression to match character repeated three times

Question

I need to simplify the following regular expression to include all the letters of the alphabet:

(a{3})|(b{3})|(c{3})|(z{3})|(A{3})|(B{3})|(C{3})|(Z{3})

In practice I want to find all the sequences of same three chars, for example:

aaa
bbb
nnn
VVV
JJJ

and so on.

Answer 1

Use backreferences. Eg. in sed:

\([a-zA-Z]\)\1\1

or in PERL regular expressions

([a-zA-Z])\1\1

Answer 2

A regular expression using backreferences would be suitable.

([a-z])\1{2}

So something along the lines of preg_match('/([a-z])\1{2}/i', $string); would suffice.