## Regular expressions

Regular expressions are enclosed in double quotes. Within them, the following are supported:

- `a` for character `a`, for any `a` (special characters need escaping).

- `\n` for the new line character (Unicode U+0A).

- `\r` for the carriage return character (Unicode U+0D).

- `\t` for the tab character (Unicode U+09).

- `\a` for character `a`, for any `a` (especially useful for escaping special characters).

- `\\` for character `\` (escaped).

- `\"` for character `"` (escaped).

- `(x)` for regular expression `x` (allows for grouping).

- `xy` for regular expression `x` followed by regular expression `y`.

- `x*` for zero or more times regular expression `x`.

- `x+` for one or more times regular expression `x`.

- `x?` for zero or one times regular expression `x`.

- `.` for any ASCII character except `\n` (new line, Unicode U+0A).

- `x|y` for either regular expression `x` or regular expression `y` (but not both).

- `[abc]` for exactly one of the characters `a`, `b` or `c`.

- `[a-z]` for exactly one of the characters `a`, `b`, …​, or `z`. This notation is called a character class. Note that the ranges of characters are based on their ASCII character codes.

- `[^a]` for any ASCII character except for character `a`. This notation is called a negated character class.

- `{s}` for the regular expression defined by shortcut `s`.

To include special characters, they must always be escaped, wherever they occur in the regular expression. For instance, regular expression `[a\^]` recognizes either character `a` or character `^` (but not both). Here the `^` character is escaped, as it is a special character (it may be used at the beginning of a character class to invert the character class).

New lines are not allowed in the regular expressions themselves. Obviously, it is possible to detect new lines using regular expressions.