This is a neat brute-force search system - it uses goroutines, one for each of the 1,200 books in the corpus, and has each one do a regex search against the in-memory text for that book.

Here's a neat trick I picked up from the source code:

    indices := fdr.rgx.FindAllStringSubmatchIndex(text, -1)

    for _, pair := range indices {
        start := pair[0]
        end := pair[1]
        leftStart := max(0, start-CONTEXT_LENGTH)
        rightEnd := min(end+CONTEXT_LENGTH, len(text))

        // TODO: this doesn't work with Unicode
        if start > 0 && isLetter(text[start-1]) {
            continue
        }

        if end < len(text) && isLetter(text[end]) {
            continue
        }

    // The '\b' word boundary regex pattern is very slow. So we don't use it here and
    // instead filter for word boundaries inside `findConcordance`.
    // TODO: case-insensitive matching - (?i) flag (but it's slow)
    pattern := regexp.QuoteMeta(keyword)

It is, indeed, impressively fast. The results seem to be sorted by first name of author. Is that a deliberate choice?

It seems to work at the word level.

Why not use a precomputed posting list?