However, there is a more subtle difference that you might run into,
which is known as greediness.
With POSIX regular expressions, matches are not greedy, which
means they will match as much as possible up until they reach some‐
thing matching the next part of the expression.
With PCRE, by default, matches are greedy, meaning they will
match as much as possible until the next part of the expression no
longer matches.
It is possible to fix this in two ways. The first is to follow quantifiers
with a question mark (?)—this will make that part of the pattern
have the same ungreedy behavior as POSIX regular expressions. The
second is to use the U modifier, which will invert the greediness,
making all quantifiers ungreedy by default and using the ? to make
them greedy.
I personally prefer to use the default behavior, as it is the default
behavior and is what most developers will expect when reading your
code.
Here we take a look at a simple regular expression for matching seg‐
ments of a URL. As you can see the code is very similar between the
POSIX-compatible regular expression and the PCREs.
Example 2-3. Migrating from POSIX- to Perl compatible regular
expressions
$url = "https://example.org/path/here";
// POSIX
$matches = [];
$regex = "^(http(s?))://(.*)$";
if (eregi($regex, $url, $matches)) {
// string matched
var_dump($matches);
/*
array(5) {
[0] =>
string(29) "https://example.org/path/here"
[1] =>
string(5) "https"
[2] =>
string(1) "s"
[3] =>
string(21) "example.org/path/here"
}
POSIX-Compatible Regular Expressions | 7
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