Basics of regular expressions

A regular expression describes a pattern of characters. Regular expressions are typically used to verify that a text value conforms to a particular pattern (such as verifying that a user-entered phone number has the proper number of digits) or to replace portions of a text value that matches a particular pattern.

Regular expressions can be simple. For example, suppose you wanted to confirm that a particular string matches "ABC," or wanted to replace every occurrence of "ABC" in a string with some other text. In that case, you could use the following regular expression, which defines the pattern consisting of the letters A, B, and C in sequence:

/ABC/

Note that the regular expression literal is delineated with the forward slash (/) character.

Regular expression patterns can also be complex, and sometimes cryptic in appearance, such as the following expression to match a valid e-mail address:

/([0-9a-zA-Z]+[-._+&])*[0-9a-zA-Z]+@([-0-9a-zA-Z]+[.])+[a-zA-Z]{2,6}/

Most commonly you will use regular expressions to search for patterns in strings and to replace characters. In those cases, you will create a regular expression object and use it as a parameter for one of several String class methods. The following methods of the String class take regular expressions as parameters: match(), replace(), search(), and split(). For more information on these methods, see Finding patterns in strings and replacing substrings.

The RegExp class includes the following methods: test() and exec(). For more information, see Methods for using regular expressions with strings.

Important concepts and terms

The following reference list contains important terms that are relevant to this feature:

Escape character
A character indicating that the character that follows should be treated as a metacharacter rather than a literal character. In regular expression syntax, the backslash character (\) is the escape character, so a backslash followed by another character is a special code rather than just the character itself.

Flag
A character that specifies some option about how the regular expression pattern should be used, such as whether to distinguish between uppercase and lowercase characters.

Metacharacter
A character that has special meaning in a regular expression pattern, as opposed to literally representing that character in the pattern.

Quantifier
A character (or several characters) indicating how many times a part of the pattern should repeat. For example, a quantifier would be used to designate that a United States postal code should contain five or nine numbers.

Regular expression
A program statement defining a pattern of characters that can be used to confirm whether other strings match that pattern or to replace portions of a string.