java.util.regex.Matcher
java.util.regex.Pattern

Comment

Pattern Fixture's fields and methods

regularExpression

input

replacement

ith

group()

replaceAll()

compile()

matcher()

find()

result()

appendReplacement()

appendTail()

groupi()

String

int

String

regularExpression A set of optional and required characters that form a pattern. The pattern may use parentheses to create groups.
compile() generates a java.util.regex.Pattern from regularExpression.
input A string that may contain a regular expression.
matcher() generates a java.util.regex.Matcher from input using the java.util.regex.Pattern from compile().
find() Searches input for a regularExpression using a java.util.regex.Matcher from matcher(). It sets ith to 0, so that groupi() starts at the first sub-group. It returns true if the search succeeds. If the match succeeds then more information can be obtained via group() and groupi().
group() Returns the entire input subsequence matched by the previous find().
groupi() Returns the ith input subsequence matched by the previous find(). It increments ith.
replacement appendReplacement() and replaceAll() generate a result(), which is a copy of input with the replacement substituted for the regularExpression.
appendReplacement() Implements a non-terminal append-and-replace step. Attempts to find(). If successful, it appends replacement to the result() and returns true.
appendTail() Implements a terminal append-and-replace step.
replaceAll() Replaces every occurrence of the regularExpression in the input with replacement.
result() returns the resulting string from appendReplacement() and appendTail()

Searching for expressions

Using group() and group(int)

Define a pattern for an apple, orange, or peach.

Pattern Fixture

regularExpression

(apple)|(orange)|(peach)

Pattern Fixture

compile?

Match against a string

Pattern Fixture

input

An apple, orange or peach a day will keep the doctor away.

Pattern Fixture

matcher?

Find each match. The group() and group(int index) functions return information about the match. group() and group(0) return the entire matching input sequence. The index corresponds to the number of left parenthesis from the beginning of the string, so

group(1) denotes apple. group(4) is undefined because the string has only 3 left parenthesis, so it throws an exception.

Pattern Fixture
find?	group?	groupi?	groupi?	groupi?	groupi?	groupi?
true	apple	apple	apple				1
true	orange	orange		orange			2
true	peach	peach			peach		3
false							4
							5

Note that the fourth row fails to find any more expressions, but the fifth row starts back over at the beginning.

Replacing expressions

Using appendReplacement() and appendTail()

Pattern Fixture
regularExpression	input	replacement
cat	one cat two cats in the yard	dog	Repace cat with dog

Pattern Fixture
compile?	matcher?

Pattern Fixture
appendReplacement?	appendReplacement?	appendReplacement?	appendTail?
one dog

Regular Expression Constructs

This example uses shows how to use regular expressions to find parameters in a SQL script. The syntax for a parameter is

a colon
whitespace optional
in, out or inout optional
and a single digit

java.util.regex.Pattern contains a full list of constructs for defining regular expressions.

Comment	:\s*(((in)?(out)?\s+)?(\d))\b
Characters	Matches	Example
:	:	:
\s*	Any number of whitespace characters:	[ \t\n\x0B\f\r]
(	Expression 1	in 1 or inout 2 or out 3 or 4
(	Expression 2 - within Expression 1	in or inout or out
(	Expression 3 - within Expression 2	in
)?	) -End ofExpression 3.? optional	see above
(	Expression 4 - within Expression 2	out
)?	) -End ofExpression 4.? optional	see above
\s+	There must be at least one space following in/out	[ \t\n\x0B\f\r]
)?	) -End ofExpression 2 (the in/out expression).? optional	see above
(\d)	Expression 5	0 1 2 3 ... 9
\b	Word Boundary - Followed by whitespace or end of string		Not 123 or 1ABC

Pattern Fixture
regularExpression	input	compile?	matcher?
:\s*(((in)?(out)?\s+)?(\d))\b	Begin : out 1 : = : 2 :in 3 :inout 4; end; :inin 5 :outout 6 :77 :8

':inin 5', ':outout6' and ':77' do not have the correct syntax.

Pattern Fixture
find?	groupi?	groupi?	groupi?	groupi?	groupi?	groupi?

Replacing with a Group Reference

Using replaceAll() and $G

replacemenent can refer to the original capturing group with a $G, where G is between 0 and 9.

Call replaceAll() with same Expression

Pattern Fixture
matcher?	replacement	replaceAll()
	$0	Begin : out 1 : = : 2 :in 3 :inout 4; end; :inin 5 :outout 6 :77 :8	Output matches input
	:$1	Begin :out 1 : = :2 :in 3 :inout 4; end; :inin 5 :outout 6 :77 :8	Strip spaces after colon
	:$5	Begin :1 : = :2 :3 :4; end; :inin 5 :outout 6 :77 :8	Strip spaces and in/out/inout