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3.3 The Backslash Character and Special Expressions

The ‘\’ character, when followed by certain ordinary characters, takes a special meaning:

\b
Match the empty string at the edge of a word.
\B
Match the empty string provided it's not at the edge of a word.
\<
Match the empty string at the beginning of word.
\>
Match the empty string at the end of word.
\w
Match word constituent, it is a synonym for ‘[_[:alnum:]]’.
\W
Match non-word constituent, it is a synonym for ‘[^_[:alnum:]]’.
\s
Match whitespace, it is a synonym for ‘[[:space:]]’.
\S
Match non-whitespace, it is a synonym for ‘[^[:space:]]’.

For example, ‘\brat\b’ matches the separate word ‘rat’, ‘\Brat\B’ matches ‘crate’ but not ‘furry rat’.