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.
