Python parser combinator for capturing text inside delimiters

Question

I'm taking a look at some parser combinator libraries in Python (Parsy to be more precise) and I'm currently faced with the following problem, simplified with a minimally working example below:

text = '''
AAAAAAAAAA AAAAAAAA AAAAAAAAAAAAAA
BBBBBBB START THE TEXT HERE SHOULD
BE CAPTURED STOP CCCCCCCCCC CCCCCC
'''

start, stop = r"STARTS?", r"STOPS?"
s = section(text, start, stop)

print(s)

which should output:

 THE TEXT HERE SHOULD 
BE CAPTURED 

The current solution I'm working is by doing a regex lookahead, it works fine, but my original problem involves combining many of these little regexes, which can get messy and a problem for others to maintain later.

from typing import Pattern, TypeVar
import re

# A Generic type declaration.
T = TypeVar("T")

def first(text: str, pattern: str, default: T, flags=0) -> T:
    """
    Given a `text`, a regex `pattern` and a `default` value, return the first match
    in `text`. Otherwise return a `default` value if no match is found.
    """
    match = re.findall(pattern, text, flags=flags)
    return match[0] if len(match) > 0 else default

def section(text: str, begin: str, end: str) -> str:
    """
    Given a `text` and two `start` and `stop` regexes, return the captured group
    found in the interval. Otherwise, return an empty string if no match is found.
    """
    return first(text, fr"{begin}([\s\S]*?)(?={end})", default="")

Parser Combinators seem to be perfect for situations like these, but I'm unable to reproduce the same behavior as the working solution, any hints would be welcome:

# A Simpler example with hardcoded stuff
from parsy import regex, seq, string

text = '''
AAAAAAAAAA AAAAAAAA AAAAAAAAAAAAAA
BBBBBBB START THE TEXT HERE SHOULD
BE CAPTURED STOP CCCCCCCCCC CCCCCC
'''

start = regex(r"STARTS?")
middle = regex(r"[\s\S]*").optional()
stop = regex(r"STOPS?")

eol = string("\n")

# Work fine
start.parse("START")
middle.parse("")
stop.parse("STOP")

section = seq(
    start,
    middle,
    stop
)
# Simpler case, breaks
section.parse("START AAA STOP")

Gives:

---------------------------------------------------------------------------
ParseError                                Traceback (most recent call last)
<ipython-input-260-fdec112e1648> in <module>
     24 )
     25 # Simpler case, breaks
---> 26 section.parse("START AAA STOP")

~/.venv/lib/python3.8/site-packages/parsy/__init__.py in parse(self, stream)
     88     def parse(self, stream):
     89         """Parse a string or list of tokens and return the result or raise a ParseError."""
---> 90         (result, _) = (self << eof).parse_partial(stream)
     91         return result
     92 

~/.venv/lib/python3.8/site-packages/parsy/__init__.py in parse_partial(self, stream)
    102             return (result.value, stream[result.index:])
    103         else:
--> 104             raise ParseError(result.expected, stream, result.furthest)
    105 
    106     def bind(self, bind_fn):

ParseError: expected 'STOPS?' at 0:14

Answer 1

The issue is that the middle parser matches the text until the end, so there is nothing for the stop parser to consume:

seq(start, middle).parse("START AAA STOP")

prints

['START', ' AAA STOP']

One solution to avoid this behavior is to use the lookahead option for the middle regex:

middle = regex(r"[\s\S]*(?=STOP)").optional()

This ensures that the matched text is followed by the "STOP" word.

Alternatively, you can use the should_fail method from Parsy:

middle = (regex(r"STOPS?").should_fail("not STOP") >> any_char).many().concat()