Is using strpos() before preg_replace() faster?

Question

Say we use this preg_replace on millions of post strings:

function makeClickableLinks($s) {
    return preg_replace('@(https?://([-\w\.]+[-\w])+(:\d+)?(/([\w/_\.#-]*(\?\S+)?[^\.\s])?)?)@', '<a href="$1" target="_blank">$1</a>', $s);
}

Asume that only 10% of all the posts contain links, would it be faster to check strpos($string, 'http') !== false before calling preg_replace()? If so, why? Doesn't preg_replace() perform some pretests internally?

Answer 1

Surprisingly, yes!

Here are benchmarks for you to analyze on 10,000,000 strings with both functions:

Test 1 - String that matches the pattern:

"Here is a great new site to visit at http://example.com so go there now!"

preg_replace alone took 10.9626309872 seconds
strpos before preg_replace took 12.6124269962 seconds ← slower

Test 2 - String that doesn't match the pattern:

"Here is a great new site to visit at ftp://example.com so go there now!"

preg_replace alone took 6.51636195183 seconds
strpos before preg_replace took 2.91205692291 seconds ← faster

Test 3 - 10% of the strings match the pattern:

"Here is a great new site to visit at ftp://example.com so go there now!" (90%)
"Here is a great new site to visit at http://example.com so go there now!" (10%)

preg_replace alone took 7.43295097351 seconds
strpos before preg_replace took 4.31978201866 seconds ← faster

It's just a simple benchmark on two strings, but there is a clear difference in speed.

---

Here is the test harness for the "10%" case:

<?php
$string1 = "Here is a great new site to visit at http://example.com so go there now!";
$string2 = "Here is a great new site to visit at ftp://example.com so go there now!";

function makeClickableLinks1($s) {
    return preg_replace('@(https?://([-\w\.]+[-\w])+(:\d+)?(/([\w/_\.#-]*(\?\S+)?[^\.\s])?)?)@', '<a href="$1" target="_blank">$1</a>', $s);
}

function makeClickableLinks2($s) {
    return strpos($s, 'http') !== false ? preg_replace('@(https?://([-\w\.]+[-\w])+(:\d+)?(/([\w/_\.#-]*(\?\S+)?[^\.\s])?)?)@', '<a href="$1" target="_blank">$1</a>', $s) : null;
}

/* Begin test harness */

$loops = 10000000;

function microtime_float() {
    list($usec, $sec) = explode(" ", microtime());
    return ((float)$usec + (float)$sec);
}

/* Test using only preg_replace */

$time_start = microtime_float();
for($i = 0; $i < $loops; $i++) {
    // Only 10% of strings will have "http"
    makeClickableLinks1($i % 10 ? $string2 : $string1);
}
$time_end = microtime_float();
$time = $time_end - $time_start;
echo "preg_replace alone took $time seconds<br/>";

/* Test using strpos before preg_replace */

$time_start = microtime_float();
for($i = 0; $i < $loops; $i++) {
    // Only 10% of strings will have "http"
    makeClickableLinks2($i % 10 ? $string2 : $string1);
}
$time_end = microtime_float();
$time = $time_end - $time_start;
echo "strpos before preg_replace took $time seconds<br/>";
?>