My research points towards "no".
Full Text Indexes Are Data About Books, and That’s Fair Use
full-text indexing is allowed as fair use under US copyright law. Indices are allowed as "transformative uses".
This source also suggests a technical way of full-text indexing without copyright infringement under two assumptions:
- You can't infringe the copyright to a work if you don't copy the work.
- If you can't reconstruct a work from its index, then distributing copies of the index doesn't infringe on the work's copyright.
However, most of the sources I could find online focus on the US and are some 10 year old. What is the latest development in this field (with focus on the EU)? Are there technical ways of avoiding an infringement of copyright?
In contrast to a metadata search that searches documents (web content, books, and other) by means of metadata (e.g. the author, date of creation, etc.) a full-text search is looking for occurrences of words, phrases, whole sentences, paragraphs, or - generally speaking - strings of characters in the document's text corpus.
A full-text index is a rather broad term for a technical solution to the problem of finding strings of characters in a text corpus without having to scan the whole document every time.
Usually, a full-text index is created by scanning the whole document and collecting statistics on the frequency of words (and so on) and often also the words' position in the document. That depends on the purpose of the index. Some indexes answer whether a words is in the document at all, others how often, or even where in the document.
To directly address the question raised by @DPenner1 in the comments: In general, indexes can identify arbitrary long phrases.
I don't know whether this is of importance but, as also the second source points out, it is sometimes technically possible to calculate (accurately) or guess the original text corpus from the index.
Once the index has been created, it is (technically) possible to copy and disseminate the index without the original text-corpus.