Basic Regular Expression Syntax

A regular expression consists of a specified set of strings that are used for matching criteria during searches.

Regular expressions can contain both special and ordinary characters. Most ordinary characters, like "A", "a", or "0", are the simplest regular expressions; they match themselves. You can concatenate ordinary characters, so last matches the string 'last'.

The following table provides a list of regular expressions:

operator	description
.	Use as a placeholder for any character. Examples: "use." matches "user", "used", "uses"
*	Zero or more instances of the previous character exist in sequence. Examples: "do.*" matches "dog", "done" since "d-o- followed by zero or more of any character". "to*" matches "to" and "too" since "t-o- followed by zero or more o’s". "fre*.." matches "frat", "free", "from" since "f-r- followed by zero or more e’s followed by any two characters".
+	One or more of the previous characters. At least one of the previous characters must be in sequence. Similar to * but more strict. Examples: "fre+.." matches "freeze", "fresh" since "f-r- followed by one or more e’s followed by any two characters".
?	Zero or one of the previous characters but not more than one. This is stricter than either (*) or (+). Examples: "ton?e" matches "toe" and "tone" since "t-o- followed by zero or one n followed by e".
?!	Negative look-ahead operator. Use as part of a construct, typically with positive look ahead. Example: "(?!dmin\$)" searches forward from current position in the sequence of characters to match "dmin$"; if found, this is a negative match and "dmin$" is exempted as a match.
?<!	Negative look-behind operator. Use as part of a construct, typically with negative look behind. Example: "(?<!c)" searches backward from current position in the sequence of characters to match "c"; if found, this is a negative match and "c" is exempted as a match.
()	Group expressions. Use to logically combine two or more patterns. Example: (cat|dog) matches "cat" and "dog".
[]	Use as a placeholder for a single character which matches any of a set of characters. Examples: "ta[pb]" matches "tap" and "tab" since "t-a- followed by one character from the set of pb". "r[aeiou]t" matches "rat", "ret", "rot", "rut" since "r- followed by one character from the set of vowels followed by t". "r[aeiou]+t" matches "rat" (plus all of the above), "riot", "root" since "r- followed by one or more vowels followed by t".
[^]	Any character not in the set. This regular expression negates the set—the character must match any character not within the set. This is a useful way of specifying a large set of characters, for instance, consonants are "not vowels". Examples: "t[^aeiou]+.*s" matches "thanks", "this" since "t- followed by one or more of any character which is not a vowel followed by zero or more of any character followed by an s".
^	Beginning of data string.
$	End of data string.
\	Escape special characters. Use to match regular expressions like * or ?. Examples: "John\.Smith" matches "John.Smith" since "John" followed by a period followed by "Smith".

To show the use of expressions, consider a couple of real-world scenarios.

Example 1: Specific Search with Inclusions and Single Exclusion

In this example, the administrator wanted to include shares that started with A through F including lowercase characters while excluding the admin$ share from the search.

The syntax for this set of expressions is as follows:

^[a-fA-F](?!dmin\$)[ \w\$]*

The first expression in this set, "^[a-fA-F]", starts the search at the beginning of the data string and searches for shares that start with "a", "b", "c", "d", "e", "f", "A", "B", "C", "D", "E", or "F". The next expression, "(?!dmin\$)", looks for the admin share to be excluded since the "!" character represents a negative match. The final expression, "[ \w\$]*", continues the entire process until the end of line is reached, or the end of the string is reached, or a white space or word is encountered.

The syntax details are as follows:

expression	description
^[a-fA-F]	Look for any character in the set a-f or A-F
(?!dmin\$)	Positive look ahead and match "dmin$"; if found, negative match
[ \w\$]*	Zero or more matches until end of line, or $, or white space, or words

Example 2: Specific Search with Inclusions and Multiple Exclusions

This example is very similar to the first example, but includes an additional exclusion. In this example, the administrator wanted to include shares that started with A through F including lowercase characters while excluding the admin$ and C$ share from the search.

The syntax for this set of expressions is:

^[a-fA-F](?!dmin\$)(?<!c)[ \w\$]*

The first expression in this set, "^[a-fA-F]", starts the search at the beginning of the data string and searches for shares that start with "a", "b", "c", "d", "e", "f", "A", "B", "C", "D", "E", or "F". The next expression, "(?!dmin\$)", looks for the admin share to be excluded since the"!" character represents a negative match. The third expression, "(?<!c)", searches backward from the current location in the string for any share starting with "c" and excludes it. The final expression, "[ \w\$]*", continues the entire process until the end of line is reached, or the end of the string is reached, or a white space or word is encountered.

The syntax details are as follows:

expression	description
^[a-fA-F]	Look for any character in the set a-f or A-F
(?!dmin\$)	Positive look ahead and match "dmin$"; if found, negative match
(?<!c)	Negative look behind and match "c"; if found, negative match
[ \w\$]*	Zero or more matches until end of line, or $, or white space, or words

This example uses specific exclusion, in that within the matches found, it then checks for and excludes shares starting with "c". Additional exemptions must each be specified with their own expression, for example, adding "(?<!B)" to this set of expressions would also exclude any share starting with B.