Regex Character Classes

In regular expressions, a character class is one or more characters enclosed in square brackets []. Character classes match any one character from the set of characters specified within the brackets.

Character classes make it easy to match a specific set of characters or exclude certain characters from a match. For example, the regular expression [aeiou] matches any vowel, while [^aeiou] matches any character that is not a vowel.

You can also use character classes to match sets of characters, such as [a-z] to match any lowercase letter or [0-9] to match any digit.

Character classes can be combined with other regular expression metacharacters, such as quantifiers and anchors, to create more complex patterns.

Note: Character classes only match a single character at a time, so if you want to match multiple characters in a row, you'll need to use a quantifier like + or *.

The followings are common examples of character classes:

• [abc] - matches any one character that is either 'a', 'b', or 'c'.
• [a-z] - matches any one lowercase letter from 'a' to 'z'.
• [A-Z] - matches any one upper case letter from 'A' to 'Z'.
• [0-9] - matches any one digit from '0' to '9'. Optionaly, use \d metacharacter.
• [^abc] - matches any one character that is not 'a', 'b', or 'c'.
• [\w] - matches any one-word character, including letters, digits, and underscore.
• [\s] - matches any whitespace character, including space, tab, and newline.
• [^a-z] - matches any one character that is not a lowercase letter from 'a' to 'z'.

The following demonstrates character classes:

The regex pattern [eor] finds all the occurrences of ‘e’, ‘o’, and ‘r’ in the text.

The regex pattern [^eor] finds all the occurrences of characters except ‘e’, ‘o’, and ‘r’ in the text.

The regex pattern [m-r] finds all the occurrences of characters that come between small m and r, that is m,n,o,p,q,r in the text.

The regex pattern [\w] finds all the occurrences of letters, digits, and underscore.

The regex pattern [\s] finds all the occurrences of white space, tab, and newline characters.

The regex pattern [^a-z] finds all the occurrences of a character that is not in a lowercase letter.

Note that we can combine one or more quantifier characters with other elements in the regex pattern.

Overall, character classes are a powerful tool in regular expressions that allow for precise and flexible pattern matching.