I used now your excellent tester at myregextester.com and my best result is:

Source text:

source_string_to_KEY1:VALUE1_match_with_pattern_KEY2:VALUE2 

Raw Match Pattern:
(?=KEY1:([^_]*))

$matches Array:
(
    [0] => 
    [1] => VALUE1
)

This solution does'nt work with IE6 (below JS 1.5) 

But I'm looking for something like:

$matches Array:
(
    [0] => VALUE1
) 

Which works with IE6 (JS 1.3)