Any regular expression that matches a single character (that
is, everything but "^" and "$") can be followed by the character
"*" to make a regular expression that matches zero or more successive
occurrences of the single character pattern. The resulting expression
is called a closure. For example, "x*" matches
zero or more x's; "xx*" matches one or more "x's"; "[a-z]*" matches
any string of zero or more lowercase letters. If there is a choice
of the number of characters to be matched, the longest possible
string is used even when a match with the null string is equally
valid. "[a-zA-Z]*" matches an entire word (which may be a null string);
"[a-zA-Z][a-zA-Z]*" matches at least an entire word (one or more
letters but not a null string); and ".*" matches a whole line (which
may be a null string). Any ambiguity in deciding which part of a
line matches an expression is resolved by choosing the match beginning
with the leftmost character, then choosing the longest possible
match at the point. So "[a-zA-Z][a-zA-Z0-9_]*" matches the leftmost
Pascal identifier on a line, "(.*)" matches anything between parentheses
(not necessarily balanced), and "..*" matches an entire line of
one or more characters but not a null string.
Character Classes (Match Any One of the Following Characters)
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