Regex for PHP associative array keys that lack quotes

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  Find with: \$([a-z0-9_]*)\[(?!('|"))([a-z0-9_]*)(?!('|"))\]
  Replace with: $\1['\3']

  \1 and \3 relate to groups in the expression that are whatever what's inside parentheses.

  The first and third group ([a-z0-9_]*) are equal and each one
  matches the variable name and the key name respectively

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Matches the following:

  $your_var_name[your_assoc_key]

Which will turn into:

  $your_var_name['your_assoc_key']

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Won't match:

  $your_var_name["your_assoc_key"] or $your_var_name['your_assoc_key']