Pattern mining from a genomic sequence
echo "&gt;ATGCTTATTGCCCATTTCGTGCATGCATATGCGCATTCGCGATCGATTAGGGATAT" | grep -oP  'TTCG[CGT][ATGC]{1,15}TTCG[CGT]'



[CGT] looks for one occurrence of one of the three.
[ATGC]{1,15} looks for "up to 15 letter long" combinations of the four bases.


Moreover, -P makes your regex call Perl-like and is required to make this regex work. Different regex flavors behave (slightly) differently.