@Override
    public String lookup(String var) {
        if (var == null) {
            return null;
        }

        final int prefixPos = var.indexOf(PREFIX_SEPARATOR);
        if (prefixPos >= 0) {
            final String prefix = toKey(var.substring(0, prefixPos));
            final String name = var.substring(prefixPos + 1);
            final StringLookup lookup = stringLookupMap.get(prefix);
            String value = null;
            if (lookup != null) {
                value = lookup.lookup(name);
            }

            if (value != null) {
                return value;
            }
        }
        if (defaultStringLookup != null) {
            return defaultStringLookup.lookup(var);
        }
        return null;
    }
Resolves the specified variable. This implementation will try to extract a variable prefix from the given
variable name (the first colon (':') is used as prefix separator). It then passes the name of the variable with
the prefix stripped to the lookup object registered for this prefix. If no prefix can be found or if the
associated lookup object cannot resolve this variable, the default lookup object will be used.

@param var the name of the variable whose value is to be looked up
@return The value of this variable or <b>null</b> if it cannot be resolved
plugin/src/main/java/io/jenkins/plugins/casc/FixedInterpolatorStringLookup.java