Regex - matching multiple nested LaTeX parameters

Question

I'm currently working on a code project in Python that transliterates LaTeX mathematical markup into standard Python commands \frac{a}{b} to a/b.

I went about this in a way that I felt would be the most friendly towards nested equations: recursion. Every equation is broken up into objects and operators, and objects, such as parenthetical statements and LaTeX terms, are evaluated again, until maximum depth is attained.

However, I've hit somewhat of a roadblock with regex when it comes to dismantling certain LaTeX terms with multiple nested parameters, like the one I mentioned above. After fiddling around and googling for an eternity, I ended up with this:

http://regex101.com/r/oO5oG9

Only problem is, I encounter this error when trying to evaluate the exact same term in Python:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python34\lib\re.py", line 206, in findall
    return _compile(pattern, flags).findall(string)
  File "C:\Python34\lib\re.py", line 288, in _compile
    p = sre_compile.compile(pattern, flags)
  File "C:\Python34\lib\sre_compile.py", line 465, in compile
    p = sre_parse.parse(p, flags)
  File "C:\Python34\lib\sre_parse.py", line 746, in parse
    p = _parse_sub(source, pattern, 0)
  File "C:\Python34\lib\sre_parse.py", line 358, in _parse_sub
    itemsappend(_parse(source, state))
  File "C:\Python34\lib\sre_parse.py", line 694, in _parse
    p = _parse_sub(source, state)
  File "C:\Python34\lib\sre_parse.py", line 358, in _parse_sub
    itemsappend(_parse(source, state))
  File "C:\Python34\lib\sre_parse.py", line 694, in _parse
    p = _parse_sub(source, state)
  File "C:\Python34\lib\sre_parse.py", line 358, in _parse_sub
    itemsappend(_parse(source, state))
  File "C:\Python34\lib\sre_parse.py", line 681, in _parse
    raise error("unexpected end of pattern")
sre_constants.error: unexpected end of pattern

I'm not quite sure what the problem is in my regex, and have been changing little things for a while trying to get it to work, to no avail...

a more simple approach consists to replace the innermost expressions first. To do that you only need to forbids nested curly brackets. — Casimir et Hippolyte
– Casimir et Hippolyte, Commented Oct 13, 2014 at 6:59

Community · Accepted Answer · 2017-05-23 12:20:28Z

1

You could eventually solve it with pyparsing. It is available via pip (see PyPI). An example how to use it is https://stackoverflow.com/a/20846900/562769.

Pyparsing makes use of formal grammars to parse strings. It is not regex, but it might be suited better to your problem.

edited May 23, 2017 at 12:20

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answered Oct 13, 2014 at 7:44

Martin Thoma

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Avinash Raj · Accepted Answer · 2014-10-13 06:51:39Z

0

Python's default re module won't support recursive calls such as (?R) or (?0) (which recurses the entire pattern). But the external regex module would support this.

>>> import regex
>>> s = "\\test{5-\\tan{66}} {8+\\frac{\\cos{2}}{1}} {\\acoth{}}"
>>> regex.findall(r'(\{(?:[^{}]|(?0))*\})', s)
['{5-\\tan{66}}', '{8+\\frac{\\cos{2}}{1}}', '{\\acoth{}}']

Source: http://www.regular-expressions.info/recurse.html

edited Oct 13, 2014 at 6:51

answered Oct 13, 2014 at 6:35

Avinash Raj

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1 Comment

Classiest Medic Over a year ago

This worked! I hope in the future, Python adds support for recursion. For now, importing this package: pypi.python.org/pypi/regex as re works just fine

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