How to write a regular expression to match a string literal where the escape is a doubling of the quote character?

I am writing a parser using ply that needs to identify FORTRAN string literals. These are quoted with single quotes with the escape 开发者_如何学Gocharacter being doubled single quotes. i.e.

'I don''t understand what you mean'

is a valid escaped FORTRAN string.

Ply takes input in regular expression. My attempt so far does not work and I don't understand why.

t_STRING_LITERAL = r"'[^('')]*'"

Any ideas?

A string literal is:

1. An open single-quote, followed by:
2. Any number of doubled-single-quotes and non-single-quotes, then
3. A close single quote.

Thus, our regex is:

r"'(''|[^'])*'"

You want something like this:

r"'([^']|'')*'"

This says that inside of the single quotes you can have either double quotes or a non-quote character.

The brackets define a character class, in which you list the characters that may or may not match. It doesn't allow anything more complicated than that, so trying to use parentheses and match a multiple-character sequence ('') doesn't work. Instead your [^('')] character class is equivalent to [^'()], i.e. it matches anything that's not a single quote or a left or right parenthesis.

It's usually easy to get something quick-and-dirty for parsing particular string literals that are giving you problems, but for a general solution you can get a very powerful and complete regex for string literals from the pyparsing module:

>>> import pyparsing
>>> pyparsing.quotedString.reString
'(?:"(?:[^"\\n\\r\\\\]|(?:"")|(?:\\\\x[0-9a-fA-F]+)|(?:\\\\.))*")|(?:\'(?:[^\'\\n\\r\\\\]|(?:\'\')|(?:\\\\x[0-9a-fA-F]+)|(?:\\\\.))*\')'

I'm not sure about significant differences between FORTRAN's string literals and Python's, but it's a handy reference if nothing else.

import re

ch ="'I don''t understand what you mean' and you' ?"

print re.search("'.*?'",ch).group()
print re.search("'.*?(?<!')'(?!')",ch).group()

result

'I don'
'I don''t understand what you mean'