Lib/regsub.py

# Regular expression subroutines:
# sub(pat, repl, str): replace first occurrence of pattern in string
# gsub(pat, repl, str): replace all occurrences of pattern in string
# split(str, pat): split string using pattern as delimiter


import regex


# Replace first occurrence of pattern pat in string str by replacement
# repl.  If the pattern isn't found, the string is returned unchanged.
# The replacement may contain references \digit to subpatterns and
# escaped backslashes.  The pattern may be a string or an already
# compiled pattern.

def sub(pat, repl, str):
	prog = compile(pat)
	if prog.search(str) >= 0:
		regs = prog.regs
		a, b = regs[0]
		str = str[:a] + expand(repl, regs, str) + str[b:]
	return str


# Replace all (non-overlapping) occurrences of pattern pat in string
# str by replacement repl.  The same rules as for sub() apply.
# Empty matches for the pattern are replaced only when not adjacent to
# a previous match, so e.g. gsub('', '-', 'abc') returns '-a-b-c-'.

def gsub(pat, repl, str):
	prog = compile(pat)
	new = ''
	start = 0
	first = 1
	while prog.search(str, start) >= 0:
		regs = prog.regs
		a, b = regs[0]
		if a == b == start and not first:
			if start >= len(str) or prog.search(str, start+1) < 0:
				break
			regs = prog.regs
			a, b = regs[0]
		new = new + str[start:a] + expand(repl, regs, str)
		start = b
		first = 0
	new = new + str[start:]
	return new


# Split string str in fields separated by delimiters matching pattern
# pat.  Only non-empty matches for the pattern are considered, so e.g.
# split('abc', '') returns ['abc'].

def split(str, pat):
	prog = compile(pat)
	res = []
	start = next = 0
	while prog.search(str, next) >= 0:
		regs = prog.regs
		a, b = regs[0]
		if a == b:
			next = next + 1
			if next >= len(str):
				break
		else:
			res.append(str[start:a])
			start = next = b
	res.append(str[start:])
	return res


# Internal subroutines:
# compile(pat): compile a pattern, caching already compiled patterns
# expand(repl, regs, str): expand \digit escapes in replacement string


# Manage a cache of compiled regular expressions.
# If the pattern is a string a compiled version of it is returned.
# If the pattern has been used before we return an already compiled
# version from the cache; otherwise we compile it now and save the
# compiled version in the cache.
# Instead of a string, a compiled regular expression can also be
# passed.
# WARNING: if the pattern syntax is changed, the cache should be
# flushed!

cache = {}

def compile(pat):
	if type(pat) <> type(''):
		return pat		# Assume it is a compiled regex
	if cache.has_key(pat):
		prog = cache[pat]	# Get it from the cache
	else:
		prog = cache[pat] = regex.compile(pat)
	return prog


# Expand \digit in the replacement.
# Each occurrence of \digit is replaced by the substring of str
# indicated by regs[digit].  To include a literal \ in the
# replacement, double it; other \ escapes are left unchanged (i.e.
# the \ and the following character are both copied).

def expand(repl, regs, str):
	if '\\' not in repl:
		return repl
	new = ''
	i = 0
	while i < len(repl):
		c = repl[i]; i = i+1
		if c <> '\\' or i >= len(repl):
			new = new + c
		else:
			c = repl[i]; i = i+1
			if '0' <= c <= '9':
				a, b = regs[eval(c)]
				new = new + str[a:b]
			elif c == '\\':
				new = new + c
			else:
				new = new + '\\' + c
	return new


# Test program, reads sequences "pat repl str" from stdin.
# Optional argument specifies pattern used to split lines.

def test():
	import sys
	if sys.argv[1:]:
		delpat = sys.argv[1]
	else:
		delpat = '[ \t\n]+'
	while 1:
		if sys.stdin.isatty(): sys.stderr.write('--> ')
		line = sys.stdin.readline()
		if not line: break
		if line[-1] == '\n': line = line[:-1]
		fields = split(line, delpat)
		if len(fields) <> 3:
			print 'Sorry, not three fields'
			print 'split:', `fields`
			continue
		[pat, repl, str] = split(line, delpat)
		print 'sub :', `sub(pat, repl, str)`
		print 'gsub:', `gsub(pat, repl, str)`