How Can I Match and Extract Nested Parentheses in Python Using PyParsing?

Python: Matching Nested Parentheses with Regex

Matching nested parentheses is a common task when dealing with mathematical expressions. However, using regular expressions provides limitations when it comes to handling nested constructs. While you may have attempted to solve this with re.compile('(. )'), it only captures the outermost parentheses.

Adopting PyParsing for Nested Constructs

Instead of using regular expressions, a more suitable approach is to employ PyParsing, a library specifically designed for parsing complex text structures. PyParsing utilizes a recursive parsing technique, allowing it to effectively handle nested constructs.

Here's an example using PyParsing:

<code class="python">import pyparsing

thecontent = pyparsing.Word(pyparsing.alphanums) | '+' | '-'
parens     = pyparsing.nestedExpr( '(', ')', content=thecontent)</code>

This defines two elements: thecontent, which matches alphanumeric characters, ' ', and '-', and parens, which represents nested expressions enclosed by parentheses.

Example Usage

To use this, you can follow this example:

<code class="python">parens.parseString("((a + b) + c)")</code>

Expected Output:

<code class="text">(
  [
    (
      ['a', '+', 'b'], {}
    ),
    '+',
    'c'
  ], {}
)</code>

Extracting Data in List Format

If you prefer to extract the nested list, you can use the following:

<code class="python">res = parens.parseString("((12 + 2) + 3)")
res.asList()</code>

Output:

<code class="text">[[['12', '+', '2'], '+', '3']]</code>

By utilizing PyParsing, you can effectively match and extract nested parentheses within complex text structures.

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