在列表元素的子集上使用lapply并返回与R中的原始长度相同的列表

I want to apply a regex operation to a subset of list elements (which are character strings) using lapply and return a list of same length as the original. The list elements are long strings (derived from reading in long text files and collapsing paragraphs into a single string). The regex operation is valid only for the subset of list elements/strings. I want the non-subsetted list elements (character strings) to be returned in their original state.I want to apply a regex operation to a subset o