Regex is tool use to find modify string and files name.
Search pattern
'word$' - $ ends with.'c.t' - any word that start with c and ends with t with only one character between.
# in conf file.
grep -v '^#' /etc/login.defs
grep '7$' /etc/login.defs
grep '\.' /etc/login.defs
grep -r 'let*' /etc/