Regular Expressions

Regular expressions can be used to find patterns in Strings, useful for validation to ensure that data is in a particular format. Compilers use regular expressions to validate the syntax of programs.

Character Class'

\d: Matches any digit
\D: Matches any non-digit
\w: Matches any word
\W: Matches any non-word
\s: Matches any whitespace
\S: Matches any non-whitespace

Quantifiers

- Matches zero or more occurrences of the preceding pattern.
- Matches one or more occurrences of the preceding pattern.
? Matches zero or one occurrences of the preceding pattern.
{n} Matches exactly n occurrences of the preceding pattern.
{n,} Matches at least n occurrences of the preceding pattern.
{n,m} Matches between n and m (inclusive) occurrences of the preceding pattern.

Examples:

. Matches any single character except a newline character
.* Matches any number of unspecified characters except newlines.
- When the applied this will match zero or more occurrences.
- causes the pattern to match one or more occurrences.
- E.g. "A" and "A+" will match "A", but "A" will match an empty string as well.
\d matches any numeric digit
[] to specify sets of characters other than those that belong to a predefined character class.
- E.g. "[aeiou]" matches any vowel.
- can be used to specify ranges of characters
- E.g. "[0-35-9]" -> 0 to 3, or 5 to 9. Not 4!
^ specifies that a pattern should match anything but.
- E.g. "[^4]" matches any non-digit and digits that is not 4.