Because special characters are the operators in regular expressions, to represent a special character as an ordinary one, escape it by preceding it with a backslash. For example, use two backslash characters (\\) to represent a backslash character.

The following rules govern regular expressions that match a single character:

• Special characters are: + * ? . [ ^ $ ( ) { | \

• Any character matches itself if it is not a special character or if a preceding backslash (\) escapes the character.

• A backslash (\) followed by any special character matches the literal character itself; that is, the backslash escapes the special character.

• A period (.) matches any character except newline.

• A set of characters enclosed in brackets ([]) is a one-character regular expression that matches any of the characters in that set. For example, “[akm]” matches an a, k, or m. If you include ] (closing square bracket) in square brackets, it must be the first character. Otherwise, it does not work, even if you use \].

• A dash can indicate a range of characters. For example, [a-z] matches any lowercase letter.

• If the first character of a set of characters in brackets is the caret (^), the expression matches any character except those characters in the set. It does not match the empty string. For example: “[^akm]” matches any character except a, k, or m. The caret loses its special meaning if it is not the first character of the set.

• You can make regular expressions case insensitive by substituting individual characters with character sets; for example, “[Nn][Ii][Cc][Kk]” is a case-insensitive pattern for the name Nick (or NICK, or nick, or even nIcK).

• You can use the following escape sequences to match specific characters or character classes:

Use the following rules to build a multicharacter regular expression:

• Parentheses group parts of regular expressions into a subexpression that can be treated as a single unit. For example, “(ha)+” matches one or more instances of ha.

• A one-character regular expression or grouped subexpression followed by an asterisk (*) matches zero or more occurrences of the regular expression. For example, “[a-z]*” matches zero or more lowercase characters.

• A one-character regular expression or grouped subexpression followed by a plus sign (+) matches one or more occurrences of the regular expression. For example, “[a-z]+” matches one or more lowercase characters.

• A one-character regular expression or grouped subexpression followed by a question mark (?) matches zero or one occurrence of the regular expression. For example, “xy?z” matches either xyz or xz.

• The carat (^) at the beginning of a regular expression matches the beginning of the field.

• The dollar sign ($) at the end of a regular expression matches the end of the field.

• The concatenation of regular expressions creates a regular expression that matches the corresponding concatenation of strings. For example, “[A-Z][a-z]*” matches any capitalized word.

• The OR character (|) allows a choice between two regular expressions. For example, “jell(y|ies)” matches either jelly or jellies.

• Curly brackets ({}) indicate a range of occurrences of a regular expression. You use them in the form “{m, n}” where m is a positive integer equal to or greater than zero indicating the start of the range and n is equal to or greater than m, indicating the end of the range. For example, “(ba){0,3}” matches up to three pairs of the expression ba. The form “{m,}” requires at least m occurrences of the preceding regular expression. The form “{m}” requires exactly m occurrences of the preceding regular expression. The form “{,n}” is not allowed.

Backreferencing lets you match text in previously matched sets of parentheses. A slash followed by a digit n (\n) refers to the nth parenthesized subexpression.

One example of how you can use backreferencing is searching for doubled words; for example, to find instances of “the the” or “is is” in text. The following example shows backreferencing in a regular expression:

(\b[A-Za-z]+)[ ]+\1

This code matches text that contains a word that is repeated twice; that is, it matches a word (specified by the \b word boundary special character and the “[A-Za-z]+)” followed by one or more spaces (specified by “[ ]+”), followed by the first matched subexpression, the first word, in parentheses. For example, it would match “is is”, but not “This is”.

ColdFusion validation normally considers a value to be valid if any of it matches the regular expression pattern. If you want to ensure that the entire entry matches the pattern, “anchor” it to the beginning and end of the field, as follows:

• If a caret (^) is at the beginning of a pattern, the field must begin with a string that matches the pattern.

• If a dollar sign ($) is at the end of a pattern, the field must end with a string that matches the pattern.

• If the expression starts with a caret and ends with a dollar sign, the field must exactly match the pattern.

The following examples show some regular expressions and describe what they match: