Regex to match unique substrings

Question

Here's a basic regex technique that I've never managed to remember. Let's say I'm using a fairly generic regex implementation (e.g., grep or grep -E). If I were to do a list of files and match any that end in either .sty or .cls, how would I do that?

Answer 1

ls | grep -E "\.(sty|cls)$"

• \. matches literally a "." - an unescaped . matches any character
• (sty|cls) - match "sty" or "cls" - the | is an or and the brackets limit the expression.
• $ forces the match to be at the end of the line

Note, you want grep -E or egrep, not grep -e as that's a different option for lists of patterns.