I want to be able to check if a Regex matches part of a string starting at a specified offset (and it doesn't need to match until the end of the string).

find searches ahead until it finds a match. I want to find a match starting at a certain position, but not necessarily all the way to the end of the string.

So are the following equivalent? -

regex.toPattern().matcher(input).region(startIndex, input.length).lookingAt()

-

regex.find(input, startIndex)?.range?.start == startIndex

Yes, that's equivalent to

lookingAt

in terms of behavior, but performance would be much worse, since it's going to be constantly trying to match stuff I'm just going to throw away.

what about the following:

fun main(args: Array<String>) {
    println("Hello, world!")
    val input = "foo456bar"
    val p1 = Regex("\\d+")
    println(p1.find(input, 3)?.range?.first == 3)
    val p2 = Regex(".{3}\\d+.+")
    println(p2.matches(input))
}

That has the same problem as

...?.start == startIndex

in that

find

will continue searching the entire string for a match where the goal is to only match at that index and avoid the performance impact of searching the entire string unnecessarily.

The

p2

approach would not do that because it's anchored but needs to skip over

{n}

read characters every time. I would assume that this is optimized to a seek, and thus comes pretty close to what you want

Oh I see. Yeah, that should do it.

@nkiesel interesting idea, but seems pretty awkward even compared to having to use Java's Pattern+Matcher. Also, in my case I'm repeatedly searching for the same Regex, but at different positions. Repeatedly modifying the Regex means a lot of regex recompilation. It seems very weird to me that Kotlin's Regex API strips out so much of the capabilities in Java's Pattern/Matcher, unless the toPattern escape hatch is used.