一段法律英语翻译(涉及美国宪法与言论自由)

阅读美国有关宪法与言论自由的书籍，碰到的一段英文，试着翻译如下：

A law constitutes an impermissible abridgment of the freedom of speech if: it regulates expression on the basis ofits content or viewpoint and is not narrowly tailored to achieve a compelling governmental interest, except that content-based regulation of non-misleading speech that proposes a lawful economic transaction is permitted if the regulation directly advances a substantial government interest that could not be advanced equally well by a less speech-restrictive regulation, and excepttoo that content-based regulation of speech is freely permitted if, inter alia,the regulated speech proposes an unlawful economic transaction or a lawfu ltransaction in a misleading way, or if it is sexually explicit and as a whole appeals to the prurient interest, and depicts or describes sexual conduct in apatently offensive way, and lacks serious artistic, political, or scientific value, or if it includes the sexually explicit depiction of children, or if the speech,by its very utterance inflicts injury or tends to incite an immediate breach of the peace; all subject to the caveat that even when speech may permissibly be regulated, if that regulation takes the form of a prior restraint on its issuance, then the regulation is ordinarily presumptively impermissible; and furthermore, a content-neutral regulation of speech is impermissible unless it is narrowly tailored to achieve a significant government interest and leaves open ample alternative channels of communication.