CODEWITHSUNDEEP
phpregexstringperformance
Madbreaks
Madbreaks

Reputation: 19539

Most efficient way to truncate a string using preg_replace?

I was looking at some code and started thinking about the most efficient way to truncate a string (in this case, a URI) using preg_replace.

First off - I realize that using preg_replace in the first place might be overkill for this task, that it may be needlessly expensive, and that it might better be handled using PHP's string-friendly functions such as substr. I do know this.

That said, consider these two different Regular Expressions:

$uri = '/one/cool/uri';    // Desired result '/one/cool'

// Using a back-reference
$parent = preg_replace('#(.*)/.*#', "$1", $uri);

// Using character class negation
$parent = preg_replace('#/[^/]+$#', '', $uri);

By default I would assume that in the former case, creating the back-reference is going to be more expensive than not doing so, and therefor the 2nd example would be preferable. But then I started wondering if using [^/] in the 2nd example might be more expensive than the corresponding . in the first example, and if so, how much more?

I prefer the first example from a readability standpoint, and since we're splitting hairs I lean towards choosing it between the two (after all, there's value in writing readable code too). May just be my personal preference though.

Thoughts?

Upvotes: 4

Views: 340

Answers (1)

Niloct
Niloct

Reputation: 10015

I also would measure running time of both options. This information from the docs may help too:

http://www.php.net/manual/en/regexp.reference.performance.php

If you are using such a pattern with subject strings that do not contain newlines, the best performance is obtained by setting PCRE_DOTALL, or starting the pattern with ^.* to indicate explicit anchoring. That saves PCRE from having to scan along the subject looking for a newline to restart at.

So, $parent = preg_replace('#^(.*)/.*#s', "$1", $uri); may speed the first option. The second one would not need this setup:

s (PCRE_DOTALL)

If this modifier is set, a dot metacharacter in the pattern matches all characters, including newlines. Without it, newlines are excluded. This modifier is equivalent to Perl's /s modifier. A negative class such as [^a] always matches a newline character, independent of the setting of this modifier.

Upvotes: 2

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