The
Pattern API contains a number of useful
predefined character classes, which offer convenient shorthands
for commonly used regular expressions:
| Construct |
Description |
. |
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] |
In the table above, each construct in the left-hand column
is shorthand for the character class in the right-hand column.
For example, \d means a range of digits (0-9), and \w
means a word character (any lowercase letter, any uppercase letter, the underscore character, or any digit).
Use the predefined classes whenever possible. They make your
code easier to read and eliminate errors introduced
by malformed character classes.
Constructs beginning
with a backslash are called escaped constructs. We previewed
escaped constructs in the
String Literals section where we mentioned the use of backslash and \Q and \E
for quotation. If you are using an
escaped construct within a string literal, you must preceed
the backslash with another backslash for the string to compile.
For example:
private final String REGEX = "\\d"; // a single digit
In this example
\d is the regular expression; the extra backslash
is required for the code to compile.
The test harness reads the expressions directly from the
Console, however, so
the extra backslash is unnecessary.
The following examples demonstrate the use of predefined
character classes.
Enter your regex: .
Enter input string to search: @
I found the text "@" starting at index 0 and ending at index 1.
Enter your regex: .
Enter input string to search: 1
I found the text "1" starting at index 0 and ending at index 1.
Enter your regex: .
Enter input string to search: a
I found the text "a" starting at index 0 and ending at index 1.
Enter your regex: \d
Enter input string to search: 1
I found the text "1" starting at index 0 and ending at index 1.
Enter your regex: \d
Enter input string to search: a
No match found.
Enter your regex: \D
Enter input string to search: 1
No match found.
Enter your regex: \D
Enter input string to search: a
I found the text "a" starting at index 0 and ending at index 1.
Enter your regex: \s
Enter input string to search:
I found the text " " starting at index 0 and ending at index 1.
Enter your regex: \s
Enter input string to search: a
No match found.
Enter your regex: \S
Enter input string to search:
No match found.
Enter your regex: \S
Enter input string to search: a
I found the text "a" starting at index 0 and ending at index 1.
Enter your regex: \w
Enter input string to search: a
I found the text "a" starting at index 0 and ending at index 1.
Enter your regex: \w
Enter input string to search: !
No match found.
Enter your regex: \W
Enter input string to search: a
No match found.
Enter your regex: \W
Enter input string to search: !
I found the text "!" starting at index 0 and ending at index 1.
In the first three examples, the regular expression is simply
. (the "dot" metacharacter) that indicates "any character."
Therefore, the match is successful in all three cases (a randomly selected
@ character, a digit, and a letter). The remaining examples each use a single regular expression construct from the
Predefined Character Classes table. You can refer to this table
to figure out the logic behind each match:
\d matches all digits
\s matches spaces
\w matches word characters
Alternatively, a capital letter means the opposite:
\D matches non-digits
\S matches non-spaces
\W matches non-word characters
