Regular Expressions

Regular expressions enable you to match a string to a pattern. The regular expression can contain a number of different elements that define the pattern. Link Conditions use regular expressions to perform pattern matching.

Wildcards

Most users are familiar with the asterisk (*) and question mark (?) wildcards available in Windows. Regular expressions provide similar functions using the dot (.) and asterisk.

To get the equivalent function of the question mark wildcard, you will need to use the dot. For example, a wildcard pattern of "Hel?o" would be equivalent to the regular expression "Hel.o".

To get the equivalent function of the asterisk wildcard, you will need to use the dot followed by an asterisk. For example, a wildcard pattern of "Opalis*" would be equivalent to the regular expression "Opalis.*".

If a dot or an asterisk is part of the text that you are searching, you will need to precede these characters using a slash (\). For example, a wildcard pattern of "myreport.*" would be the equivalent to the regular expression "myreport\..*".

Advanced Regular Expressions

To build regular expressions you will need to create an expression that contains the text that you are searching for and special characters that create a pattern which describes how the text that you are searching for will appear.

 

Special Character	Meaning
.	Matches any character except a newline.
*	Matches the preceding item 0 or more times. For example, the "a*" pattern will match any string of a's in a row "a", "aaa", "aaaaaaaaaaaa" as well as an empty string "" will all match. To match any string of any character, use a dot followed by an asterisk. For example "a.*" will match any text that begins with the letter "a" and ends with any string of characters such as "abbb", "abcdef", or "automatic restart".
+	Matches the preceding item 1 or more times. This is like * but you must have a least 1 of the preceding item to make a match. For example, the "ab+" pattern will match "abbbbb", "ab", but won't match "a". To contrast, the "ab*" pattern will match "a".
?	Matches the preceding item 0 or 1 time. For example, the "ab?" pattern will match "a" or "ab" but won't match "abbb".
|	Matches either the preceding expression or the following expression. Logical OR operator.
$	Matches the expression at the end of the input or line.  For example, "ab$" will match "I took a cab" or "drab" but won't match "absolutely not".
^	Matches the expression at the beginning of the input or line. For example, "^ab" will match "absolutely not" or "abacuses are great!" but won't match "I took a cab" or "drab".
\	For characters that are usually treated as special. This indicates that the next character is literal and is not to be treated as a special character. For example, "\." means match the "." character and not just any character.
[ ]	A character set. Matches any one of the enclosed characters. You can specify a range of characters by using a hyphen. For example, [a-zA-Z] matches any letter of the alphabet.
[^ ]	An excluded character set. This is the opposite of []. If any of the characters inside the brackets exist, the regular expression match fails. You can specify a range of characters by using a hyphen. For example, [^a-zA-Z] makes sure that none of the letters in the alphabet are present.
( )	A group expression. This groups an expression into an item that you can apply special characters to. For example, "a*(ba)+" will match "ba" "aba" or "ababa" but won't match "abbba" or "abaa"

Examples

 

Expression	Meaning
[a-zA-Z]+	The text contains only letters of the alphabet.
^\*	The text begins with an asterisk.
(abc|def)$	The end of the text is either "abc" or "def".
Ha..y	The text begins with "Ha" followed by any two characters followed by a "y".
Help.*	The text is "Help" followed by any number of other characters.