Regular Expressions - Refactoring HTML: Improving the Design of Existing Web Applications - page 309

Quantifiers
A period or a literal character by itself always matches exactly one character. However, you can append a
quantifier to it to indicate that the character may appear a variable number of times.
Zero or One: ?
A normal character suffixed with a question mark indicates that the character appears only optionally (zero
times or once). For example, the regular expression a?b matches ab and b . The a is optional. The regular
expression a?b?c? matches abc , ab , bc , ac , bc , a , b , and c , as well as the empty string.
You can suffix a period with a question mark to indicate that any character may or may not appear. For
example, the regular expression 200.? matches 200 as well as 2000 , 2001 , 200Z , and 200! .
Zero or More: *
An asterisk (*) suffix indicates that the preceding character appears zero or more times. For example, a*b
matches ab , aaab , aaaaab , and b . However, it does not match abb or acb .
You can put an asterisk after a period to indicate that any number of any characters may appear. For example,
a.*b matches ab , aaab , aaaaab , abb , acb , a123b , and "a quick brown fox jumped into the tub" .
Unlike UNIX shell globs, the asterisk alone does not match anything. It must be suffixed to something else. For
example, to list all the HTML files in the current working directory, you'd usually type something such as this:
$ ls *html
However, in most regular-expression dialects, the regular expression that matches all strings ending in html is
.*html . *html without the initial period is a syntax error.
One or More: +
A plus sign (+) suffix indicates that a character appears one or more times. For example, a+b matches ab , aaab ,
and aaaaab . However, it does not match a single b , abb , or acb .
Of course, a plus sign after a period indicates that one or more of any character is required. The regular
expression a.+b requires at least one character between the a and the b , so it matches aaab , aaaaab , abb , acb ,
and a123b but not a simple ab .
A Specific Number of Times: {}
You can specify that a character must appear a specified number of times using curly braces. For example, a{3}
is the same as the pattern aaa . It stands for exactly three a 's in a row.
You can also specify a range of possible occurrences using a comma. A{3,5} allows three to five A s in a row.
That is, it matches AAA , AAAA , and AAAAA but not AA or AAAAAA .
You can omit the second, maximum value to indicate that at least a certain number of repetitions is required
but more are allowed. For example, a{3,} matches aaa , aaaa , aaaaa , aaaaaa , and any larger sequence of a 's.
Table A.4 shows some more examples.