Regular Expression, also known as Regex, are used for text matching. Regular expressions contain a series of characters that search for data in text.

The java.util.regex package provides the following classes for regular expressions:

• Matcher class: It is used to perform match operations on character sequences.
• Pattern class: It is used to define a search pattern.
• PatternSyntaxException class: It is used to give syntax error for a regular expression.

A. Find range of Characters:

Character	Meaning
[0-9]	Find character in range 0-9
[xyz]	Find any one character x, y, or z
[^xyz]	Find any character other than x, y, or z

B. Metacharacters:

Character	Meaning
\b	Search for a word match
\d	Searches a digit
\D	Searches a non-digit
\s	Searches whitespace
\S	Searches non-whitespace
\w	Searches for word characters 0-9, a-z, A-Z
\W	Searches non-word character
$	Searches for a match at the end of the string
a|b	Searches for any one of the given word patterns, i.e., a or b
.	Search just one instance of any character
^	Searches for a match at the start of the string

C. Quantifiers:

Character	Meaning
a?	Matches zero or one occurrences of string having a
a+	Matches at least one occurrence of string having a
a*	Matches zero or more occurrences of string having a
a{b}	Matches string having a sequence of b a’s
a{b,c}	Matches string having a sequence of b to c a’s
a{b,}	Matches string having a sequence of at least b a’s