匹配xa？b？c的正则表达式？但不是单独的x

I'm trying to write a regex that matches xa?b?c? but not x. In reality, 'x', 'a', 'b', and 'c' are not single characters, they are moderately complex sub-expressions, so I'm trying to avoid something like x(abc|ab|ac|bc|a|b|c). Is there a simple way to match "at least one of a, b, and c, in that order" in a regex, or am I out of luck?I'm trying to write a regex that matches xa?b?c