perl regular expression

letz say that my file has 7 records with only one field. So my file has:

11111111
000000000000000
1111
aaaabbbccc
1111111222000000
aaaaaaaa
zz

All i need is:

  1. when the field has a repetition of the same instance(a-z or 0-9), i would consideer it to be invalid. eg.(11111111,000000000000000,1111,aaaaaaaa,zz are all invalid)

2.If there is a repetition of 2 or more instances,it is valid. (eg. 1111111222000000,aaaabbbccc is valid)

  1. Please note that the length of each field may vary.

So, i am expecting the output:
aaaabbbccc
1111111222000000

I am not sure if the title of the thread suits the matter. Anyway, whoever crosses this thread i would appreciate your input. Thanks in advance!

Duplicate of multipe instance scenario in perl