List of Regular Expressions Symbols

 Character Classes

Character classes are used to define the content of the pattern. E.g. what should the pattern look for?
.   Dot, any character (may or may not match line terminators, read on)

\d   A digit: [0-9]
\D   A non-digit: [^0-9]
\s   A whitespace character: [ \t\n\x0B\f\r]
\S   A non-whitespace character: [^\s]
\w   A word character: [a-zA-Z_0-9]
\W   A non-word character: [^\w]

However; notice that in Java, you will need to “double escape” these backslashes.
String pattern = "\\d \\D \\W \\w \\S \\s";

Quantifiers

Quantifiers can be used to specify the number or length that part of a pattern should match or repeat. A quantifier will bind to the expression group to its immediate left.

*      Match 0 or more times
+      Match 1 or more times
?      Match 1 or 0 times
{n}    Match exactly n times
{n,}   Match at least n times
{n,m}  Match at least n but not more than m times

Meta-characters

Meta-characters are used to group, divide, and perform special operations in patterns.

\   Escape the next meta-character (it becomes a normal/literal character)
^   Match the beginning of the line
.   Match any character (except newline)
$   Match the end of the line (or before newline at the end)
|   Alternation (‘or’ statement)
()   Grouping
[]   Custom character class