\number
Matches the contents of the group of the same number. Groups are numbered starting from 1. For example, (.+) \1 matches 'the the' or '55 55', but not 'the end' (note the space after the group). This special sequence can only be used to match one of the first 99 groups. If the first digit of number is 0, or number is 3 octal digits long, it will not be interpreted as a group match, but as the character with octal value number. Inside the "[" and "]" of a character class, all numeric escapes are treated as characters.

\A
Matches only at the start of the string.

\b
Matches the empty string, but only at the beginning or end of a word. A word is defined as a sequence of alphanumeric or underscore characters, so the end of a word is indicated by whitespace or a non-alphanumeric, non-underscore character. Note that \b is defined as the boundary between \w and \ W, so the precise set of characters deemed to be alphanumeric depends on the values of the UNICODE and LOCALE flags. Inside a character range, \b represents the backspace character, for compatibility with Python's string literals.

\B
Matches the empty string, but only when it is not at the beginning or end of a word. This is just the opposite of \ b, so is also subject to the settings of LOCALE and UNICODE.

\d
Matches any decimal digit; this is equivalent to the set [0-9].

\D
Matches any non-digit character; this is equivalent to the set [^0-9].

\s
Matches any whitespace character; this is equivalent to the set [ \t\n\r\f\v].

\S
Matches any non-whitespace character; this is equivalent to the set [^ \t\n\r\f\v].

\w
When the LOCALE and UNICODE flags are not specified, matches any alphanumeric character and the underscore; this is equivalent to the set [a-zA-Z0-9_]. With LOCALE, it will match the set [0-9_] plus whatever characters are defined as alphanumeric for the current locale. If UNICODE is set, this will match the characters [0-9_] plus whatever is classified as alphanumeric in the Unicode character properties database.

\W
When the LOCALE and UNICODE flags are not specified, matches any non-alphanumeric character; this is equivalent to the set [^a-zA-Z0-9_]. With LOCALE, it will match any character not in the set [0-9_], and not defined as alphanumeric for the current locale. If UNICODE is set, this will match anything other than [0-9_] and characters marked as alphanumeric in the Unicode character properties database.

\Z
Matches only at the end of the string.