Regular Expressions: Basic Character Classes

• Some sets of characters occur together frequently. For example, we often need to recognize numbers (the digits 0-9). We can use \d for that. It recognizes 0, 1, ..., 8, 9.

• We use \s to recognize whitespace: spaces, newlines (\n), tabs (\t), etc.

• There are more of these, but we'll start with \s and \d. These are called character classes.

• >

  /\s/.test(' ');

  Result:

  true

• >

  /\s/.test('\n');

  Result:

  true

• >

  /\s/.test('');

  Result:

  false

• >

  /\s/.test('x\ty'); /* \t is a tab character */

  Result:

  true

• >

  /\s/.test('cat');

  Result:

  false

• >

  /\d/.test('0');

  Result:

  true

• >

  /\d/.test('9');

  Result:

  true

• >

  /\d/.test('a');

  Result:

  false

• Making these character classes upper-case negates them. For example, \d is all digits. But \D is any character that isn't a digit.

• >

  /\D/.test('0');

  Result:

  false

• >

  /\S/.test(' ');

  Result:

  false

• >

  /\S/.test('0');

  Result:

  true

• A character class matches only one character in the string. If we want to match multiple characters, we can use + or *.

• >

  /^\d$/.test('1000');

  Result:

  false

• >

  /^\d*$/.test('3000');

  Result:

  true

• >

  /^\d*$/.test('1000 cats');

  Result:

  false

• >

  /^\d*$/.test('cats');

  Result:

  false

• >

  /^\d*$/.test('');

  Result:

  true