Exluce data from brackets using regex (preg_match_all)

Exluce data from brackets using regex (preg_match_all)

Input string:
:txt{sometext}:alpha
I want to extract data like this (extracted from brackets):
Result using preg_match_all():
sometext
Trying like this, but none of this works:
php > preg_match_all('/^(\:txt)(.*)+(\{)(.*)+(\})/i',
':txt{sometext}:alpha', $m); var_dump($m);
array(6) {
[0] =>
array(1) {
[0] =>
string(14) ":txt{sometext}"
}
[1] =>
array(1) {
[0] =>
string(1) ":"
}
[2] =>
array(1) {
[0] =>
string(0) ""
}
[3] =>
array(1) {
[0] =>
string(1) "{"
}
[4] =>
array(1) {
[0] =>
string(0) ""
}
[5] =>
array(1) {
[0] =>
string(1) "}"
}
}
Note: as sample I have like this :txt{sometext}:alpha:another{mydata}, so
I can extract data from :another and give results like mydata.