How do I match nested braces using regular expressions in PHP? - php

Question

I have an LaTeX document I want to match. And I need a RegEx match that matches the following:
\ # the backslash in the beginning
[a-zA-Z]+ #a word
(\{.+\})* # any amount of {something}
However, and her is the catch;
In the last line, it 1. needs to be greedy and 2. needs to have a matching number of {} inside itself.
Meaning if I have the string \test{something\somthing{9}}
it would match the whole. And it needs to be in that order ({}). So that it doesn't match the following:
\LaTeX{} is a document preparation system for the \TeX{}
just
\LaTeX{}
and
\TeX{}
Help anyone? Maybe someone have an better idea for matching? Should I not use regular expressions?

Answer 1

This can be done with recursion:
$input = "\LaTeX{} is a document preparation system for the \TeX{}
\latex{something\somthing{9}}";
preg_match_all('~(?<token>
\\\\ # the slash in the beginning
[a-zA-Z]+ #a word
(\{[^{}]*((?P>token)[^{}]*)?\}) # {something}
)~x', $input, $matches);
This correctly matches \LaTeX{}, \TeX{}, and \latex{something\somthing{9}}

Answer 2

PHP could be used since it supports recursive regex-matching. But, as I said, if you have comments in your LaTeX-like strings that can have { or } in them, this will fail.
A demo:
$text = 'This is a \LaTeX{ foo { bar { ... } baz test {} done } } document
preparation system for the \TeX{a{b{c}d}e{f}g{h}i}-y people out there';
preg_match_all('/\\\\[A-Za-z]+(\{(?:[^{}]|(?1))*})/', $text, $matches, PREG_SET_ORDER);
print_r($matches);
which produces:
Array
(
[0] => Array
(
[0] => \LaTeX{ foo { bar { ... } baz test {} done } }
[1] => { foo { bar { ... } baz test {} done } }
)
[1] => Array
(
[0] => \TeX{a{b{c}d}e{f}g{h}i}
[1] => {a{b{c}d}e{f}g{h}i}
)
)
A quick explanation:
\\\\ # the literal '\'
[A-Za-z]+ # one or more letters
( # start capture group 1 <-----------------+
\{ # the literal '{' |
(?: # start non-capture group A |
[^{}] # any character other than '{' and '}' |
| # OR |
(?1) # recursively match capture group 1 ---+
) # end non-capture group A
* # non-capture group A zero or more times
} # the literal '}'
) # end capture group 1

Answer 3

Unfortunately, I believe this is impossible. Bracket matching (detecting properly paired, nested brackets) is commonly used as an example of a problem that cannot be solved with a finite state machine, such as a regular expression parser. You could do it with a context free grammar, but that's just not how regex works. Your best solution is to use a regex like {*[^{}]*}* for the initial check, and then another short script to check whether it's an even number.
In conclusion: don't try and do it with only regex. This is not a problem that can be solved with regex alone.