This is very much related to https://stackoverflow.com/questions/546433/regular-expression-to-match-outer-brackets however, I specifically want to know how or whether it&#39;s possible to do this [**regex&#39;s recursive pattern**](https://pypi.python.org/pypi/regex)? *I&#39;m yet to find a python example using this strategy so think this ought to be a useful question!*

I&#39;ve [seen](http://www.regular-expressions.info/recurse.html) [some](https://stackoverflow.com/a/12455760/1240268) [claims](https://stackoverflow.com/a/19122223/1240268) [that](http://www.catonmat.net/blog/recursive-regular-expressions/) recursive patterns can be used to match balanced parenthesis, but no examples using python&#39;s [regex](https://pypi.python.org/pypi/regex) package (Note: re **does not** support recursive pattern, you need to use regex).

One [claim](http://www.regular-expressions.info/recurse.html) is that syntax is `b(?:m|(?R))*e` where:

&gt; `b` is what begins the construct, `m` is what can occur in the middle of the construct, and `e` is what can occur at the end of the construct

---

I want to extract matches for the *outer* braces in the following:

    &quot;{1, {2, 3}} {4, 5}&quot;
    [&quot;1, {2, 3}&quot;, &quot;4, 5&quot;]  # desired

Note that this is easy to do the same for *inner* braces:

    re.findall(r&quot;{([^{}]*)}&quot;, &quot;{1, {2, 3}} {4, 5}&quot;)
    [&#39;2, 3&#39;, &#39;4, 5&#39;]

*(In my example I was using finditer (over match objects), see [here](https://github.com/hayd/ctox/blob/a0e8a4068c5fae5fc26dbf14d63ca4f42cc84670/ctox/subst.py#L133).)*



So I had hoped that the following, or some variation, would work:

    regex.findall(r&quot;{(:[^{}]*|?R)}&quot;, &quot;{1, {2, 3}} {4, 5}&quot;)
    regex.findall(r&quot;({(:[^{}]*|?R)})&quot;, &quot;{1, {2, 3}} {4, 5}&quot;)
    regex.findall(r&quot;({(:.*|(?R))*})&quot;, &quot;{1, {2, 3}} {4, 5}&quot;)
    regex.findall(r&quot;({(:.*)|(?R)*})&quot;, &quot;{1, {2, 3}} {4, 5}&quot;)
    regex.findall(r&quot;({(:[^{}])|(?R)})&quot;, &quot;{1, {2, 3}} {4, 5}&quot;)

but I&#39;m scuppered by either [] or `error: too much backtracking`.


**Is it possible to extract match objects for the outer parenthesis using regex&#39;s recursion?**

---

Obviously, I run the risk of being shot down with:

- [don&#39;t parse html with regex](https://stackoverflow.com/a/1732454/1240268)
- do this with pyparse
- write a proper lexer &amp; parser e.g. using ply

I want to emphasis this is about **how to use the recursive pattern** (which if my understanding is correct, takes us outside of regular language parsing, so may can actually be possible!). If it can be done, this ought to be a cleaner solution.

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Recursive pattern in regex