Question

I can do this using explode (or have fun with with strrpos), but i prefer to using preg_replace because should be a bit faster, i think (isn’t it?). Plus, it’s concise and elegant.

The purpose is, given a string like a_b_c to obtain another sting where characters following the last _ where substituted with passed string.

I’m not good at regular expression. I’ve to find the time to buy a good book at study. Anyway i’ve tried this regex '/_(.*)$/' as match the end of the string, capture any character following the last underscore.

What’s wrong in my argumentation?

// Do it using explode
function foo($string, $replacement)
{
    $pieces = explode('_', $string);
    array_pop($pieces);
    return implode('_', array_merge($pieces, array($replacement)));
}

// Do it using regular expression (not working)
function bar($string, $replacement)
{
    return preg_replace('/_(.*)$/', $replacement, $string);
}

echo foo('a_b_c', 3); // Prints a_b_3
echo bar('a_b_c', 3); // Prints a3 wrong!!!

Answer 1

Depending on your common search string and your version of PCRE, either preg_replace or strrpos will probably fare best:

Functions

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);
}

Test harness

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;
}

Results

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

Note that (at least on my set-up with PHP 5.4.0), preg_replace gives strrpos a run for its money until there are a large number of underscores preceding the last one.

EDIT: I plugged bfrohs’s regex into the suite and unless the underscore to replace is near the beginning of the string, it doesn’t do very well:

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