
Contents > Developing ColdFusion MX Applications > Using Regular Expressions in Functions > Using backreferences

Using backreferences

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:

REReplace("There is is coffee in the the kitchen",
"[ ]+([A-Za-z]+)[ ]+\1"," * ","ALL")

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:

There * coffee in * kitchen

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, etc. The next section includes an example using multiple backreferences.


Contents > Developing ColdFusion MX Applications > Using Regular Expressions in Functions > Using backreferences

Version 6.1

Comments are no longer accepted for ColdFusion MX 6.1. ColdFusion 8 is the current version.

Comments

f i g i t a l said on Sep 30, 2005 at 7:06 AM :

Much appreciated if you give a few more examples of backreferencing here. This seems different that other RE implementations I have used. Powerful stuff (if you can figure it out!).

Thanks,

Scott

RSS feed | Send me an e-mail when comments are added to this page | Comment Report

Current page: http://livedocs.adobe.com/coldfusion/6.1/htmldocs/regexp14.htm