Question

I’m (finally) starting to learn regex, and I’m wondering if there’s any notable difference between these two pattern strings. I’m trying to match lines such as “Title=Blah“, and match “Title” and “Blah” in two groups.

The problem comes with titles like “Title=The = operator“. Here are the two choices to solve the problem:

^([^=]+)=(.+)$
^(.+?)=(.+)$

Is there any difference between the two, either performance-wise or functionality-wise?

Answer 1

The first one requires that there be at least one non-= character before an = in order to match, whereas the second doesn’t; it’ll match on a leading ==.

Depending on your content, the first one could run significantly faster. Here’s why:

An Alternative to Laziness
In this
case, there is a better option than
making the plus lazy. We can use a
greedy plus and a negated character
class: <[^=]+>. The reason why this is
better is because of the backtracking.
When using the lazy plus, the engine
has to backtrack for each character in
the HTML tag that it is trying to
match. When using the negated
character class, no backtracking
occurs at all when the string contains
valid HTML code. Backtracking slows
down the regex engine. You will not
notice the difference when doing a
single search in a text editor. But
you will save plenty of CPU cycles
when using such a regex repeatedly in
a tight loop in a script that you are
writing…