Understanding Regular Expressions

Proxy Server allows you to use regular expressions to identify resources. Regular expressions specify a pattern of character strings. In the proxy server, regular expressions are used to find matching patterns in URLs.

The following example shows a regular expression:

[a-z]*://[^:/]*\.abc\.com.*

This regular expression would match any documents from the .abc.com domain. The documents could be of any protocol and could have any file extension.

The following table lists the regular expressions and their corresponding meanings.

Table 16–2 Regular expressions and their meanings

Expression	Meaning
.	Matches any single character except a newline.
x?	Matches zero or one occurrences of regular expression x.
x*	Matches zero or more occurrences of regular expression x.
x+	Matches one or more occurrences of regular expression x.
x{n,m}	Matches the character x where x occurs at least n times but no more than m times.
x{n,}	Matches the character x where x occurs at least n times.
x{n}	Matches the character x where x occurs exactly n times.
[abc]	Matches any of the characters enclosed in the brackets.
[^abc]	Matches any character not enclosed in the brackets.
[a-z]	Matches any characters within the range in the brackets.
x	Matches the character x where x is not a special character.
\x	Removes the meaning of special character x.
"x"	Removes the meaning of special character x.
xy	Matches the occurrence of regular expression x followed by the occurrence of regular expression y.
x|y	Matches either the regular expression x or the regular expression y.
^	Matches the beginning of a string.
$	Matches the end of a string.
(x)	Groups regular expressions.

This example illustrates how you can use some of the regular expressions in Understanding Regular Expressions.

[a-z]*://([^.:/]*[:/]|.*\.local\.com).*

• [a-z]* matches a document of any protocol.

• :// matches a (:) followed by (//).

• [^.:/]*[:/] matches any character string that does not include a (.),(:) or (/), and is followed by either a (:) or a (/). This expression therefore matches host names that are not fully qualified and hosts with port numbers.

• |.*\.local\.com does not match fully qualified domain name host names such as local.com but does match documents in the .local.com domain.

• .* matches documents with any file extension.

As noted in Understanding Regular Expressions, the backslash can be used to escape or remove the meaning of special characters. Characters such as the period and question mark have special meanings, and therefore, must be escaped if they are used to represent themselves. The period, in particular, is found in many URLs. So, to remove the special meaning of the period in your regular expression, you need to precede it with a backslash.