Extended regular expressions
Basic regular expressions are sufficient for most purposes, but
a more sophisticated form of regular expression is available known
as an extended regular expression or
ERE. There are a couple of principal extra
features available using EREs that are unavailable to BREs. The
symbol +, following a bracket expression (or single
character or dot) indicates one or more consecutive occurrences of
the expression, in the same way that * indicates zero
or more. The symbol ?, in the same context, indicates
zero or one occurrences of that expression, so
[[:alpha:]]+[[:digit:]]?
matches any string commencing with a letter, consisting only of
letters, and terminated optionally by a single digit. Note that the
? is not to be confused with the ? in
pattern matching. If two EREs are separated by a |
(vertical bar), the result matches either of those two EREs.
Parentheses may be used to group subexpressions together:
(xyz|ab)\.c
will match either xyz.c or ab.c, and
no other string. If you need a parenthesis to be a matched
character in an ERE you must escape it.
Worked example
Write an ERE to match any string consisting either of only
upper-case letters or only lower-case letters.
Solution: As in the previous worked example, the
expression will commence with ^ and end with $. By taking advantage
of the symbol + a match for upper-case letters would be
[[:upper:]]+ and [[:lower:]]+ for
lower-case letters. A sequence of letters of the same case will be
matched by ([[:upper:]]+|[[:lower:]]+)), and a
solution is therefore:
^[[:lower:]]+|[[:upper:]]+$
|