Thanks to everyone who responded. The query was in fact obscure since I didn't use the <code> tag. Please find the query in a proper format below.

I have a file called names.txt with the following content
john[JM]mcgroddy
stephen[SG]gomsey
yuri[YA]alchenko

I was trying to extract the initials placed between special characters  [and ] and output this to a new file initials.txt as

JM
SG
YA

.

I hope that I have clarified the question. Thanks once again to those who guided me the correct regex. Regards, PS.