Strings, Characters and Regular Expressions - Java: How to Program Early Objects - page 631

Method replaceFirst (line 33) replaces the first occurrence of a pattern match. Java
String s are immutable; therefore, method replaceFirst returns a new String in which
the appropriate characters have been replaced. This line takes the original String and
replaces it with the String returned by replaceFirst . By iterating three times we replace
the first three instances of a digit ( \d ) in secondString with the text "digit" .
Method split divides a String into several substrings. The original is broken in any
location that matches a specified regular expression. Method split returns an array of
String s containing the substrings between matches for the regular expression. In line 39,
we use method split to tokenize a String of comma-separated integers. The argument is
the regular expression that locates the delimiter. In this case, we use the regular expression
",\\s*" to separate the substrings wherever a comma occurs. By matching any white-
space characters, we eliminate extra spaces from the resulting substrings. The commas and
white-space characters are not returned as part of the substrings. Again, the Java String
",\\s*" represents the regular expression ,\s* . Line 40 uses Arrays method toString to
display the contents of array results in square brackets and separated by commas.
Classes Pattern and Matcher
In addition to the regular-expression capabilities of class String , Java provides other class-
es in package java.util.regex that help developers manipulate regular expressions. Class
Pattern represents a regular expression. Class Matcher contains both a regular-expression
pattern and a CharSequence in which to search for the pattern.
CharSequence (package java.lang ) is an interface that allows read access to a
sequence of characters. The interface requires that the methods charAt , length , subSe-
quence and toString be declared. Both String and StringBuilder implement interface
CharSequence , so an instance of either of these classes can be used with class Matcher .
Common Programming Error 14.2
A regular expression can be tested against an object of any class that implements interface
CharSequence , but the regular expression must be a String . Attempting to create a reg-
ular expression as a StringBuilder is an error.
If a regular expression will be used only once, static Pattern method matches can
be used. This method takes a String that specifies the regular expression and a CharSe-
quence on which to perform the match. This method returns a boolean indicating
whether the search object (the second argument) matches the regular expression.
If a regular expression will be used more than once (in a loop, for example), it's more
efficient to use static Pattern method compile to create a specific Pattern object for
that regular expression. This method receives a String representing the pattern and
returns a new Pattern object, which can then be used to call method matcher . This
method receives a CharSequence to search and returns a Matcher object.
Matcher provides method matches , which performs the same task as Pattern method
matches , but receives no arguments—the search pattern and search object are encapsu-
lated in the Matcher object. Class Matcher provides other methods, including find ,
lookingAt , replaceFirst and replaceAll .
Figure 14.24 presents a simple example that employs regular expressions. This pro-
gram matches birthdays against a regular expression. The expression matches only birth-
days that do not occur in April and that belong to people whose names begin with "J" .