How do I parse this string?

I would like to parse the following string

a='4:256404:val=9.999e+20:val=1.82:val=2.15:val=2.66:val=2.18:val=1.81';

to obtain the vector x = [9.999e+20 1.82 2.15 2.66 2.18 1.81]. In reality the string is 5308 characters long and there are very many of them. I believe I can't parse this with regexp because I want to separate with a defined sequence ':val='. I could do the following, but is there a cleaner way?

    matchThisString = ':val=';
    iBreak = findstr(a, matchThisString);
    x = NaN*ones(1, (length(iBreak)-1));
    for ii = 1:(length(iBreak)-1)
        iStart = iBreak(ii) + length(matchThisString);
        iEnd = iBreak(ii + 1) - 1;
        x(ii) = str2num(a(iStart:iEnd));
    end

Answer 1

[Azzi Abdelmalek]

a='9.999e+20:val=1.82:val=2.15:val=2.66:val=2.18:val=1.81'
b=str2double(regexp(a,':val=','split'))

[K E]

Great! I thought I could not use regexp here because it was a specific sequence, but turns out I can!