cpython/Lib/regex_syntax.py

# These bits are passed to regex.set_syntax() to choose among
# alternative regexp syntaxes.

# 1 means plain parentheses serve as grouping, and backslash
#   parentheses are needed for literal searching.
# 0 means backslash-parentheses are grouping, and plain parentheses
#   are for literal searching.
RE_NO_BK_PARENS = 1

# 1 means plain | serves as the "or"-operator, and \| is a literal.
# 0 means \| serves as the "or"-operator, and | is a literal.
RE_NO_BK_VBAR = 2

# 0 means plain + or ? serves as an operator, and \+, \? are literals.
# 1 means \+, \? are operators and plain +, ? are literals.
RE_BK_PLUS_QM = 4

# 1 means | binds tighter than ^ or $.
# 0 means the contrary.
RE_TIGHT_VBAR = 8

# 1 means treat \n as an _OR operator
# 0 means treat it as a normal character
RE_NEWLINE_OR = 16

# 0 means that a special characters (such as *, ^, and $) always have
#   their special meaning regardless of the surrounding context.
# 1 means that special characters may act as normal characters in some
#   contexts.  Specifically, this applies to:
#	^ - only special at the beginning, or after ( or |
#	$ - only special at the end, or before ) or |
#	*, +, ? - only special when not after the beginning, (, or |
RE_CONTEXT_INDEP_OPS = 32

# Now define combinations of bits for the standard possibilities.
RE_SYNTAX_AWK = (RE_NO_BK_PARENS | RE_NO_BK_VBAR | RE_CONTEXT_INDEP_OPS)
RE_SYNTAX_EGREP = (RE_SYNTAX_AWK | RE_NEWLINE_OR)
RE_SYNTAX_GREP = (RE_BK_PLUS_QM | RE_NEWLINE_OR)
RE_SYNTAX_EMACS = 0

# (Python's obsolete "regexp" module used a syntax similar to awk.)