Parsing a comma-separated "key: value" string

Parsing a comma-separated "key: value" string

I have a text file with several blocks with several lines that may look
like this:
{ key1: value, key2: value,
key3: value,
key4: value, key5: value }
{ key1: value, key2: value, key3: value,
key4: value, key5: value }
Given a key, how can I get all the corresponding values? Note that neither
the key names nor the values have a fixed length, blocks start and finish
with braces and pairs are separated by commas.
My first try was with grep and cut, but I couldn't get all the keys. I
guess that this should be easy with sed or awk, but their syntax confuses
me a lot.