You use parenthesis to group components of a regular expression into subexpressions. For example, the regular expression "(ha)+" matches one or more occurrences of the string "ha". ColdFusion performs an additional operation when using subexpressions; it automatically saves the characters in the search string matched by a subexpression for later use within the regular expression. Referencing the saved subexpression text is called backreferencing.  You can use backreferencing when searching for repeated words in a string, such as "the the" or "is is". The following example uses backreferencing to find all repeated words in the search string and replace them with an asterisk:

Using this regular expression, ColdFusion detects the two occurrences of "is" as well as the two occurrences of "the", replaces them with an asterisk enclosed in spaces, and returns the following string:

You interpret the regular expression [ ]([A-Za-z])[ ]+\1 as follows: Use the subexpression ([A-Za-z]) to search for character strings consisting of one or more letters, enclosed by one or more spaces, [ ], followed by the same character string that matched the first subexpression, \1.  You reference the matched characters of a subexpression using a slash followed by a digit n (n) where the first subexpression in a regular expression is referenced as \1, the second as \2, and so on. The next section includes an example using multiple backreferences.

You can use backreferences in the replacement string of both the  REReplace and  REReplaceNoCase  functions. For example, to replace the first repeated word in a text string with a single word, use the following syntax:

This results in the sentence:

You can use the optional fourth parameter to REReplace, scope, to replace all repeated words, as in the following code:

This results in the following string:

The next example uses two backreferences to reverse the order of the words "apples" and "pears" in a sentence:

In this example, you reference the subexpression (apples) as \1 and the subexpression (pears) as \2. The REReplace function returns the string:

The  REReplace and  REReplaceNoCase  functions support special characters in replacement strings to convert replacement characters to uppercase or lowercase. The following table describes these special characters:

To include a literal \u, or other code, in a replacement string, escape it with another backslash.

For example, the following statement replaces the uppercase string "HELLO" with a lowercase "hello". This example uses backreferences to perform the replacement.

The result of this example is the string "Don't shout\scream hello".

You use the backslash character, \, to escape backreference and case-conversion characters in replacement strings. For example, to include a literal "\u" in a replacement string, escape it, as in "\u".

By default, a set of parentheses will both group the subexpression and capture its matched text for later referral by backreferences. However, if you insert "?:" as the first characters of the subexpression, ColdFusion performs all operations on the subexpression except that it will not capture the corresponding text for use with a back reference. This is useful when alternating over subexpressions containing differing numbers of groups would complicate backreference numbering. For example, consider an expression to insert a "Mr." in between Bonjour|Hi|Hello and Bond, using a nested group for alternating between Hi & Hello:

This example returns "Hello Mr. Bond". If you did not prohibit the capturing of the Hi/Hello group, the \2 backreference would end up referring to that group instead of " Bond", and the result would be "Hello Mr.ello".