? - Match An Element Zero Or One Time from the Linux command line by OnWorks

Skip to content

? - Match An Element Zero Or One Time

This quantifier means, in effect, “Make the preceding element optional.” Let’s say we wanted to check a phone number for validity and we considered a phone number to be valid if it matched either of these two forms:

(nnn) nnn-nnnn nnn nnn-nnnn

where “n” is a numeral. We could construct a regular expression like this:

^$?[0-9][0-9][0-9]$? [0-9][0-9][0-9]-[0-9][0-9][0-9][0-9]$

In this expression, we follow the parentheses characters with question marks to indicate that they are to be matched zero or one time. Again, since the parentheses are normally

metacharacters (in ERE), we precede them with backslashes to cause them to be treated as literals instead.

Let’s try it:

[me@linuxbox ~]$ echo "(555) 123-4567" | grep -E '^\(?[0-9][0-9][0-9]

\)? [0-9][0-9][0-9]-[0-9][0-9][0-9][0-9]$'

(555) 123-4567

[me@linuxbox ~]$ echo "555 123-4567" | grep -E '^$?[0-9][0-9][0-9]$

? [0-9][0-9][0-9]-[0-9][0-9][0-9][0-9]$'

555 123-4567

[me@linuxbox ~]$ echo "AAA 123-4567" | grep -E '^$?[0-9][0-9][0-9]$

? [0-9][0-9][0-9]-[0-9][0-9][0-9][0-9]$'

[me@linuxbox ~]$

[me@linuxbox ~]$ echo "(555) 123-4567" | grep -E '^\(?[0-9][0-9][0-9]