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Patterns which start with negative lookbehind assertions may match at the beginning of the string being searched.
Matches the empty string, but only at the beginning or end of a word.
Empty matches are included in the result unless they touch the beginning of another match.
Exception raised when a string passed to one of the functions here is not a valid regular expression (for example, it might contain unmatched parentheses) or when some other error occurs during compilation or matching.
The solution is to use Python’s raw string notation for regular expression patterns; backslashes are not handled in any special way in a string literal prefixed with is a one-character string containing a newline.
Usually patterns will be expressed in Python code using this raw string notation.
The module defines several functions, constants, and an exception.
Some of the functions are simplified versions of the full featured methods for compiled regular expressions.
(Incompatibility note: in the original Python 1.5 release, if the tuple was one element long, a string would be returned instead.
Most non-trivial applications always use the compiled form.
This flag allows you to write regular expressions that look nicer and are more readable by allowing you to visually separate logical sections of the pattern and add comments.
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 formally, is used to represent word boundaries, and means “backspace” only inside character classes.) Octal escapes are included in a limited form: If the first digit is a 0, or if there are three octal digits, it is considered an octal escape. As for string literals, octal escapes are always at most three digits in length.
Thus, complex expressions can easily be constructed from simpler primitive expressions like the ones described here.