How can you use extended regular expressions with grep or sed without using -E or -r?

• A. Backslash the offending characters such as (, ), {, }
• B. Use the --extended option
• C. You cannot
• D. Use a different tool

Answer: (A)

When grep and sed run in basic regular expression mode by default, you can still express extended regex patterns by escaping the metacharacters that implement grouping, alternation, and repetition. By prefixing those characters with a backslash, you activate their extended meaning inside the basic engine. So you would write grouping as ( … ), alternation as |, and repetition as {m,n}. This is why backslashes before those symbols lets you achieve extended regex behavior without using -E or -r.

For example, grep '(cat|dog)' file matches either cat or dog. To require a sequence of three a's, use grep 'a{3}' file. In sed, you can do something like sed 's/(foo|bar)/baz/g' to replace either foo or bar with baz.