Alpha

 [[:alpha:]]

 [[:alnum:]]

• ack, ag, grep, egrep, fgrep, agrep, ngrep, pgrep, awk, sed, strings, tr, tail, mtail, git grep, wc, uniq, LogQL, findstr (Windows), rg, git-grep, cut
• Regular expression, PCRE, pcre_compile, grok, find . -regex, ReDoS