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copied!<p>Depending on your common search string and your version of PCRE, either <code>preg_replace</code> or <code>strrpos</code> will probably fare best:</p> <h2>Functions</h2> <pre><code>function usingExplode($string, $replacement) { $pieces = explode('_', $string); array_pop($pieces); return implode('_', array_merge($pieces, array($replacement))); } function usingStrrpos($string, $replacement) { return substr($string, 0, strrpos($string, '_') + 1) . $replacement; } function usingPreg($string, $replacement) { return preg_replace('/_[^_]*$/', '_' . $replacement, $string); } </code></pre> <h2>Test harness</h2> <pre><code>function speedTest($function, $string, $count = 100000) { $start = microtime(true); for ($i = 0; $i < $count; $i++) { $function($string, 'replacement'); } $end = microtime(true); printf('%-12s: %01.2fs%s', $function, $end - $start, PHP_EOL); } $tests = array('a_b_c', 'abcdefghijklmnopqrstuvwxy_z', 'a_bcdefghijklmnopqrstuvwxyz', 'a_b_c_d_e_f_g_h_i_j_k_l_m_n_o_p_q_r_s_t_u_v_w_x_y_z'); foreach ($tests as $test) { echo $test . ':' . PHP_EOL; speedTest('usingExplode', $test); speedTest('usingStrrpos', $test); speedTest('usingPreg', $test); echo PHP_EOL; } </code></pre> <h2>Results</h2> <pre><code>a_b_c: usingExplode: 0.64s usingStrrpos: 0.34s usingPreg : 0.33s abcdefghijklmnopqrstuvwxy_z: usingExplode: 0.61s usingStrrpos: 0.32s usingPreg : 0.32s a_bcdefghijklmnopqrstuvwxyz: usingExplode: 0.60s usingStrrpos: 0.32s usingPreg : 0.32s a_b_c_d_e_f_g_h_i_j_k_l_m_n_o_p_q_r_s_t_u_v_w_x_y_z: usingExplode: 1.39s usingStrrpos: 0.32s usingPreg : 0.71s </code></pre> <p>Note that (at least on my set-up with PHP 5.4.0), <code>preg_replace</code> gives <code>strrpos</code> a run for its money until there are a large number of underscores preceding the last one.</p> <p><strong>EDIT</strong>: I plugged <a href="https://stackoverflow.com/a/10236047/283078">bfrohs's regex</a> into the suite and unless the underscore to replace is near the beginning of the string, it doesn't do very well:</p> <pre><code>a_b_c: usingPreg2: 0.40s abcdefghijklmnopqrstuvwxy_z: usingPreg2: 1.91s a_bcdefghijklmnopqrstuvwxyz: usingPreg2: 0.38s a_b_c_d_e_f_g_h_i_j_k_l_m_n_o_p_q_r_s_t_u_v_w_x_y_z: usingPreg2: 1.04s </code></pre>

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