Lib/re/__init__.py (part 5)

Source:

cpython 3.14 @ ab2d84fe1023/Lib/re/__init__.py ↗

This annotation covers substitution and the regex flag system. See module_re4_detail for re.compile, re.match, re.search, re.findall, and Pattern objects.

Map

Lines	Symbol	Role
1-80	re.fullmatch	Match pattern against entire string
81-160	re.sub / re.subn	Replace matches with a string or callable
161-240	re.escape	Escape special regex characters
241-360	Regex flags	re.IGNORECASE, re.MULTILINE, re.DOTALL, etc.
361-500	re.error	Regex compilation and match errors

Reading

re.sub

# CPython: Lib/re/__init__.py:220 sub
def sub(pattern, repl, string, count=0, flags=0):
    """Return the string obtained by replacing the leftmost non-overlapping
    occurrences of pattern in string by the replacement repl."""
    return _compile(pattern, flags).sub(repl, string, count)

re.sub(r'\d+', 'N', 'a1b22c') returns 'aNbNc'. When repl is a callable, it receives the Match object and returns the replacement string. Backreferences like \1 in repl are expanded.

re.escape

# CPython: Lib/re/__init__.py:320 escape
def escape(pattern):
    """Escape special characters in pattern."""
    # ASCII letters, digits, and '_' are not escaped
    _special_chars_map = {
        ord(c): '\\' + c
        for c in r'\.^$*+?{}[]|()'}
    return pattern.translate(_special_chars_map)

re.escape('1+1=2') returns '1\\+1\\=2'. Used when you want to match a literal string that may contain regex metacharacters: re.compile(re.escape(user_input)).

Regex flags

# CPython: Lib/re/__init__.py:60 flags
class RegexFlag(enum.IntFlag):
    A = ASCII     = 256    # a  — ASCII-only matching
    I = IGNORECASE = 2     # i  — case-insensitive
    L = LOCALE    = 4      # l  — locale-dependent
    M = MULTILINE = 8      # m  — ^ and $ match line boundaries
    S = DOTALL    = 16     # s  — . matches newline
    U = UNICODE   = 32     # u  — Unicode matching (default)
    X = VERBOSE   = 64     # x  — allow whitespace and comments
    N = NOFLAG    = 0

re.compile(r'(?i)hello') is equivalent to re.compile('hello', re.IGNORECASE). Inline flags (?imsx) apply only to the part of the pattern after them (or the whole pattern if at the start).

gopy notes

re.sub is module/re.Sub in module/re/module.go. It calls pattern.ReplaceAllStringFunc (Go regexp) when repl is a callable, or pattern.ReplaceAllString for string replacements. re.escape uses regexp.QuoteMeta. Flags are stored as a uint32 bitmask and translated to Go regexp options.