Regular Expressions in Java

String Methods that accept Regular Expressions
Methods	Description
+matches (regex: String): boolean	Returns true if this string matches the pattern.
+replaceAll (regex: String, replacement: String): String	Returns a new string that replaces all matching substrings with the replacement.
+replaceFirst (regex: String, replacement: String): String	Returns a new string that replaces the first matching substring with the replacement.
+split (regex: String): String[]	Returns an array of strings consisting of the substrings split by the matches.
+split (regex: String, limit: int): String[]	Same as the preceding split method except that the limit parameter controls the number of times the pattern is applied.

A regular expression is a way of denoting a pattern. In Java this pattern is denoted by a String within the delimiters double quotes ("). Here are some definitions to start with:

A word character (\w) is any letter (upper or lower case), digit, or underscore.
A digit character (\d) is any digit in the range 0 through 9.
A line terminator character is normally not visible in a document. But is embedded in a document to indicate the end of a line. These characters are newline or line feed (\n), carriage return (\r), and form feed (\f).
A white space character also is not displayed but takes up space in display of a document. The white space characters include just a single blank space ' ', the tab character (\t), and the line terminators listed above. The white space character is denoted by \s.
The back slash is a special character that starts an escape sequence like \n, \t, \s. To use an escape sequence in a Java regular expression two back slashes have to be used like so "\\s".
A quantifier is a special character or characters that specifies how many times the pattern before the quantifier repeats itself. Like . (period), *, +. Their meanings are explained below.
You may use parentheses to group patterns.

Regular Expressions and Their Matches
Regular Expression	Matches
. (period)	any character other than a line terminator
(ab\|cd)	ab or cd
[abc]	a, b, or c
[^abc]	any character except a, b, or c.
[a-z]	any character from a to z (inclusive).
\d	a digit same as [0-9]
\D	not a digit, same as [^0-9]
\w	a word character
\W	not a word character
\s	a white space character
\S	a non white space character
\b	word boundary
\B	not a word boundary
^p	begins with pattern p
p$	ends with pattern p
p*	zero or more occurrences of pattern p
p+	one or more occurrences of pattern p
p?	zero or one occurrence of pattern p
p{n}	exactly n occurrences of pattern p
p{n,}	at least n occurrences of pattern p
p{n,m}	between n and m occurrences of pattern p (inclusive)