RegExp pattern to capture around two-characters delimiter - regex

Question

I have a string which is something like:
prefix::key0==value0::key1==value1::key2==value2::key3==value3::key4==value4::
I want to retrieve the value associated to a key (say, key1). The following pattern:
::key1==([^:]*)
...will work only if there are no ':' character in the value, so I want to make sure the pattern matching will stop only for the substring ::, but I'm can't find how to do that, as most examples I see are about single character matching.
How do I modify the regexp pattern to match all characters between "::key1==" and the next "::" ?
Thanks!

Answer 1

Can you do something like this : ::key1==(.*?)::? Assuming the language supports the lazy ? operator, this should work.

Answer 2

As mentioned in my comment to your question, if the entirety of your string is
prefix::key0==value0::key1==value1::key2==value2::key3==value3::key4==value4::
I would suggest exploding/splitting the string at :: instead of using regex as it will usually always be faster. You didn't specify language but here is a php example:
// string
$string = "prefix::key0==value0::key1==value1::key2==value2::key3==value3::key4==value4::";
// explode using :: as delimiter
$string = explode('::',$string);
// for each element...
foreach ($string as $value) {
// check if it has == in it
if (strpos($value,'==')!==false) $matches[] = $value;
}
// output
echo "<pre>";print_r($matches);
output:
Array
(
[0] => key0==value0
[1] => key1==value1
[2] => key2==value2
[3] => key3==value3
[4] => key4==value4
)
However, if you insist on the regex approach, here negative look-ahead alternative
::((?:(?!::).)+)
php example
// string
$string = "prefix::key0==value0::key1==value1::key2==value2::key3==value3::key4==value4::";
preg_match_all('~::((?:(?!::).)+)~',$string,$matches);
//output
echo "<pre>";print_r($matches);
output
Array
(
[0] => key0==value0
[1] => key1==value1
[2] => key2==value2
[3] => key3==value3
[4] => key4==value4
)

Answer 3

I think you're looking for a positive look-ahead:
::key0==(.*?)(?=::\w+==)
With the following:
prefix::key0==val::ue0::key1==value1::key2==value2::key3==value3::key4==value4::
It correctly finds val::ue0. This also assumes the keys conform to \w ([0-9A-Za-z_])
Also, a positive look-ahead may be a bit of overkill, but will work if the answer contains ::, too.