Expanding Chemical Equations - my answer to StackOverflow question: http://stackoverflow.com/questions/20670653/php-function-to-expand-chemical-equations

chem.php

<?php
class chem_expand {

	protected $multiplier;

	function compute_bracketed_replacement( $groups ) {
		// In-case if invalid input, output "<error>" in string.
		$result = '<error>';

		// If we have "(Chem)Multiplier"
		if ( !empty( $groups[1] ) && !empty( $groups[2] ) ) {
			// Keep multiplier
			$this->multiplier = intval( $groups[2] );

			// Look for "Chem" Or "ChemCount".
			$result           = preg_replace_callback( '/([A-Z][a-z]*)(\d+)?/mx',
													   array( $this, 'multiply_digits_replacement' ), $groups[1] );
		} elseif ( !empty( $groups[3] ) ) {
			// Just plain bracketed "(anything here)".
			$result = $groups[3];
		}

		return $result;
	}

	function multiply_digits_replacement( $groups ) {
		// "Chem"
		$result = $groups[1];

		// Assume only one.
		$count = 1;

		// Count.
		if ( !empty( $groups[2] ) ) {
			$count = intval( $groups[2] );
		}

		// Multiply the count out.
		$count = ( $count * $this->multiplier );
		if ( $count > 1 ) {
			// More than one, need the new count in the string.
			$result = $result . $count;
		}

		return $result;
	}

	function test() {
		echo '<p>starting test</p>';
		$test_values = array(
			'(NH4)3'             => 'N3H12',
			'(Mo3O10)4'          => 'Mo12O40',
			'(NH4)3(P(Mo3O10)4)' => 'N3H12PMo12O40',
			'H((O)2)2'           => 'HO4'
		);

		foreach ( $test_values as $input => $expected ) {
			$actual = $this->expand( $input );
			if ( $actual !== $expected ) {
				echo '<p>failure</p>';
				echo '<p>Actual: \"' . $actual . '"</p>';
				echo '<p>Expected: \"' . $expected . '"</p>';

				return;
			}
		}

		echo '<p>success</p>';
	}

	function expand( $subject ) {

		// Expand the inner brackets first.
		// Loop through all the inner "(Formula)Multiplier" first, then simple bracketed "(anything)".
		$output = preg_replace_callback(
			'/\(
			((?!\()[^()]+)
			\)
			(\d+)
			|
			\(
			((?!\()[^()]+)
			\)/ix',
			array( $this, 'compute_bracketed_replacement' ), $subject );

		// If we actually changed the content, then call ourselves again and expand the brackets further.
		if ( $output !== $subject ) {
			$output = $this->expand( $output );
		}

		return $output;
	}

}

$instance = new chem_expand();
$instance->test();