[COFF] Re: Requesting thoughts on extended regular expressions in grep.

Thank you all for very interesting and engaging comments & threads to chase / pull / untangle. I'd like to expand / refine my original question a little bit. On 3/2/23 11:54 AM, Grant Taylor via COFF wrote: While some reading of the references that Clem provided I came across multiple indications that back-references can be problematic from a performance stand point. So I'd like to know if all back-references are problematic, or if very specific back-references are okay. Suppose I have the following two lines: aaa aaa aaa bbb Does the following RE w/ back-reference introduce a big performance penalty? (aaa|bbb) \1 As in: % echo "aaa aaa" | egrep "(aaa|bbb) \1" aaa aaa I can easily see how a back reference to something that is not a fixed length can become a rabbit hole. But I'm wondering if a back-reference to -- what I think is called -- an alternation (with length fixed in the RE) is a performance hit or not. Now to read and reply to the many good comments that people have shared. :-) -- Grant. . . . unix || die