If you browse through the
Pattern class specification, you'll see tables summarizing
the supported regular expression constructs.
In the "Character Classes" section you'll find the following:
| Construct |
Description |
[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) |
[a-d[m-p]] |
a through d, or m through p: [a-dm-p] (union) |
[a-z&&[def]] |
d, e, or f (intersection) |
[a-z&&[^bc]] |
a through z, except for b and c: [ad-z] (subtraction) |
[a-z&&[^m-p]] |
a through z, and not m through p: [a-lq-z] (subtraction) |
The left-hand column specifies the regular expression constructs,
while the right-hand column describes the conditions under which
each construct will match.
Note: The word "class" in the phrase "character class" does not refer to
a .class file. In the context of regular
expressions, a character class is a set of characters
enclosed within square brackets. It specifies the
characters that will successfully match a single character
from a given input string.
Simple Classes
The most basic form of a character class is to simply place a set of
characters side-by-side within square brackets.
For example, the regular expression
[bcr]at will match the words "bat", "cat", or "rat" because
it defines a character class
(accepting either "b", "c", or "r") as its first character.
Enter your regex: [bcr]at
Enter input string to search: bat
I found the text "bat" starting at index 0 and ending at index 3.
Enter your regex: [bcr]at
Enter input string to search: cat
I found the text "cat" starting at index 0 and ending at index 3.
Enter your regex: [bcr]at
Enter input string to search: rat
I found the text "rat" starting at index 0 and ending at index 3.
Enter your regex: [bcr]at
Enter input string to search: hat
No match found.
In the above examples, the overall match succeeds only when the first letter
matches one of the characters defined by the character class.
Negation
To match all characters
except those listed,
insert the "
^" metacharacter at the beginning of the character class. This technique is known as
negation.
Enter your regex: [^bcr]at
Enter input string to search: bat
No match found.
Enter your regex: [^bcr]at
Enter input string to search: cat
No match found.
Enter your regex: [^bcr]at
Enter input string to search: rat
No match found.
Enter your regex: [^bcr]at
Enter input string to search: hat
I found the text "hat" starting at index 0 and ending at index 3.
The match is successful only if the first character of the input string
does
not contain any of the characters defined by the character class.
Ranges
Sometimes you'll want to define a character class that includes
a range of values, such as the letters "a through h" or the
numbers "1 through 5".
To specify a range, simply insert the "
-"
metacharacter between the
first and last character to be matched, such as
[1-5] or
[a-h].
You can also place different ranges beside each other
within the class to further expand the match possibilities.
For example,
[a-zA-Z] will match any letter of the alphabet:
a to z (lowercase) or A to Z (uppercase).
Here are some examples of ranges and negation:
Enter your regex: [a-c]
Enter input string to search: a
I found the text "a" starting at index 0 and ending at index 1.
Enter your regex: [a-c]
Enter input string to search: b
I found the text "b" starting at index 0 and ending at index 1.
Enter your regex: [a-c]
Enter input string to search: c
I found the text "c" starting at index 0 and ending at index 1.
Enter your regex: [a-c]
Enter input string to search: d
No match found.
Enter your regex: foo[1-5]
Enter input string to search: foo1
I found the text "foo1" starting at index 0 and ending at index 4.
Enter your regex: foo[1-5]
Enter input string to search: foo5
I found the text "foo5" starting at index 0 and ending at index 4.
Enter your regex: foo[1-5]
Enter input string to search: foo6
No match found.
Enter your regex: foo[^1-5]
Enter input string to search: foo1
No match found.
Enter your regex: foo[^1-5]
Enter input string to search: foo6
I found the text "foo6" starting at index 0 and ending at index 4.
Unions
You can also use
unions to create a single character class
comprised of two or more separate character classes.
To create a union, simply nest one class inside the other,
such as
[0-4[6-8]]. This particular union creates a
single character class that matches the numbers 0, 1, 2, 3, 4, 6, 7, and 8.
Enter your regex: [0-4[6-8]]
Enter input string to search: 0
I found the text "0" starting at index 0 and ending at index 1.
Enter your regex: [0-4[6-8]]
Enter input string to search: 5
No match found.
Enter your regex: [0-4[6-8]]
Enter input string to search: 6
I found the text "6" starting at index 0 and ending at index 1.
Enter your regex: [0-4[6-8]]
Enter input string to search: 8
I found the text "8" starting at index 0 and ending at index 1.
Enter your regex: [0-4[6-8]]
Enter input string to search: 9
No match found.
Intersections
To create a single character class matching only the characters common to all of its
nested classes, use
&&, as in
[0-9&&[345]]. This particular intersection creates a single character class matching only
the numbers common to both character classes: 3, 4, and 5.
Enter your regex: [0-9&&[345]]
Enter input string to search: 3
I found the text "3" starting at index 0 and ending at index 1.
Enter your regex: [0-9&&[345]]
Enter input string to search: 4
I found the text "4" starting at index 0 and ending at index 1.
Enter your regex: [0-9&&[345]]
Enter input string to search: 5
I found the text "5" starting at index 0 and ending at index 1.
Enter your regex: [0-9&&[345]]
Enter input string to search: 2
No match found.
Enter your regex: [0-9&&[345]]
Enter input string to search: 6
No match found.
And here's an example that shows the intersection of two ranges:
Enter your regex: [2-8&&[4-6]]
Enter input string to search: 3
No match found.
Enter your regex: [2-8&&[4-6]]
Enter input string to search: 4
I found the text "4" starting at index 0 and ending at index 1.
Enter your regex: [2-8&&[4-6]]
Enter input string to search: 5
I found the text "5" starting at index 0 and ending at index 1.
Enter your regex: [2-8&&[4-6]]
Enter input string to search: 6
I found the text "6" starting at index 0 and ending at index 1.
Enter your regex: [2-8&&[4-6]]
Enter input string to search: 7
No match found.
Subtraction
Finally, you can use
subtraction to negate one or more nested character classes, such as
[0-9&&[^345]].
This example creates a single character class that matches everything from 0 to 9,
except the numbers 3, 4, and 5.
Enter your regex: [0-9&&[^345]]
Enter input string to search: 2
I found the text "2" starting at index 0 and ending at index 1.
Enter your regex: [0-9&&[^345]]
Enter input string to search: 3
No match found.
Enter your regex: [0-9&&[^345]]
Enter input string to search: 4
No match found.
Enter your regex: [0-9&&[^345]]
Enter input string to search: 5
No match found.
Enter your regex: [0-9&&[^345]]
Enter input string to search: 6
I found the text "6" starting at index 0 and ending at index 1.
Enter your regex: [0-9&&[^345]]
Enter input string to search: 9
I found the text "9" starting at index 0 and ending at index 1.
Now that we've covered how character classes are created,
You may want to review the
Character Classes table
before continuing with the next section.
