replacing a substring of a string with another string

[yahn]

How can I use string.replace() to do something like:

string str, str2, str3;
str  = "an adequate amount of text";
str2 = "amount of ";
st3 = "";

str.replace(str2, str3); // an adequate text

//or
str  = "an adequate amount of text";
str3 = "[how did this get here?]";

str.replace(str2, str3); // an adequate [how did this get here?] text

From looking at the documentation, it seems that replace() will not replace more than the amount of text it's replacing.