# Special character sequences

Printable characters, such as “a” or “b”, are defined by simply typing them into a regular expression. In addition, there are some shorthands for common non-printable characters and character classes.

Special character sequences are listed in the following table:

Table 1. Special character sequences
Sequence | Description
--- | ---
`\a` | Bell (BEL) = `\x07`
`\t` | Horizontal tab (HT) = `\x09`
`\n` | Linefeed (LF) = `\x0A`
`\f` | Formfeed (FF) = `\x0C`
`\r` | Carriage return (CR) = `\x0D`
`\e` | Escape (ESC) = `\x1B`
`\OOO` | Octal code `OOO` of the character.
`\xHH` | Hexadecimal code `HH` of the character. Case-insensitive. For example, "`\xaa`" is regarded to be the same as "`\xAA`".
`\c<char>` | Control character that corresponds to `Ctrl+<char >`, where `<char>` is an uppercase letter.
`\w` | "word" class character = `[A-Za-z0-9_]`
`\W` | Non-"word" class character = `[^A-Za-z0-9_]`
`\s` | Whitespace character = `[ \t\r\n\f]`
`\S` | Non-whitespace character = `[^ \t\r\n\f]`
`\d` | Digit character = `[0-9]`
`\D` | Non-digit character = `[^0-9]`
`\b` | Backspace (BS) = `\x08` Note: Allowed only in bracket expressions.
`\Q` `<expr>` `\E` | Quotes all metacharacters between `\Q` and `\E`. Backslashes are regarded as normal characters. For example, "`\QC:\file.exe\E`" matches the "`C:\file.exe`" string, not the "`C:\x0Cile.exe`" string, where `\x0C` is the formfeed "`\f`".

## Example of using special character sequences

    # This fingerprint matches HTTP content
    # for which the length is >= 10000
    # The situation context for this regular expression could be either
    # "HTTP Request Header Line" or "HTTP Reply Header Line"
    Content-Length: \d\d\d\d\d

    # The regular expression could be also written as shown below
    Content-Length: \d{5}