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Java: Basic Regular expression summary

Matching a single character

Characters that otherwise have special regexp meanings

\ Precedes characters that have a special meaning: \. \+ \* \? \| \{ \( \[ \^ \$

Characters that need to be written in a special way

\t The tab character
\n The newline (line feed) character
\r The carriage-return character
\f The form-feed character

Matching a single character with a predefined character class

. Any character (may or may not match line terminators)
\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]

Defining Character classes (match one character)

Character classes provide a way to specify a set of characters. The class specification is enclosed in []. The set can also be expressed by what must not be in it by beginning the set with a caret, "^". Minus, "-", can be used to indicate a range of character values. Altho a character class matches only one character, a quantifier following it can be used to match multiple characters.
[abc] a, b, or c (simple class)
[^abc] Any character except a, b, or c (negation)
[a-zA-Z] a through z or A through Z, inclusive (range)

Position and Boundary patterns (match zero characters)

^ The beginning of a line. Very useful.
$ The end of a line. Very userful. ^$ matches all emtpy lines.
\b A word boundary
\B A non-word boundary
\A The beginning of the input
\G The end of the previous match
\Z The end of the input but for the final terminator, if any
\z The end of the input

Quantifiers (repeating the previous element)

 
Greedy quantifiers - Expand as much as possible
X? X, once or not at all
X* X, zero or more times
X+ X, one or more times
X{n} X, exactly n times
X{n,} X, at least n times
X{n,m} X, at least n but not more than m times
 
Reluctant quantifiers - Expand only if forced by later failure to match
X?? X, once or not at all
X*? X, zero or more times
X+? X, one or more times
X{n}? X, exactly n times
X{n,}? X, at least n times
X{n,m}? X, at least n but not more than m times

Other

 
Alternation
X|Y Tries matching X first, if that doesn't work, tries Y
 
Grouping - Parentheses both group and create a numbered element that can be used later.
(X) X. This capturing group is remembered so it can be referenced later. Numbered starting at 1.
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