# Character Classes

Domains:

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 d, e, or f (intersection)
[a-z&&[^bc 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 input string to search: cat
I found the text "cat" starting at index 0 and ending at index 3.

Enter input string to search: rat
I found the text "rat" starting at index 0 and ending at index 3.

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 input string to search: cat
No match found.

Enter input string to search: rat
No match found.

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 input string to search: b
I found the text "b" starting at index 0 and ending at index 1.

Enter input string to search: c
I found the text "c" starting at index 0 and ending at index 1.

Enter input string to search: d
No match found.

Enter input string to search: foo1
I found the text "foo1" starting at index 0 and ending at index 4.

Enter input string to search: foo5
I found the text "foo5" starting at index 0 and ending at index 4.

Enter input string to search: foo6
No match found.

Enter input string to search: foo1
No match found.

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 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: 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: 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 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: 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: 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: 4
I found the text "4" starting at index 0 and ending at index 1.

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. 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: 3
No match found.

Enter input string to search: 5
No match found.

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.            

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