Regex: Match everything except backreference

Question

I have the following example lines:

a_a
b_c

How (using grep/egrep) would I match the lines where the first letter

Answer 1

Use group matches.

There you create a group with the 1st character, and then put group (\1) at the end.

^(.).*\1$

egrep '^(.).*\1$'  

Opposite:

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

pcregrep '^(.).*((?!\1).)+$'

Answer 2

Use a negative look behind anchored to end of input. A general regex for "first char is not last char" is:

^(.).*(?<!\1)$

To match only your type of input:

^(.)_.(?<!\1)$

Answer 3

May be you are looking for this:

\b([a-z])\w+(?!(\1))([a-z])\b

it works for:

a_a and b_c

(tested with: The Regulator 2.0.3)

You can then adjust this regex to meet further your needs