Could we add `fun String.startsWith(` `prefix:...

# stdlib

[nkiesel]

Could we add fun String.startsWith(
prefix: Regex,
ignoreCase: Boolean = false
): Boolean (and also endsWith)? The ignoreCase parameter would be debatable, but since the String variant has it and it's easy to add to the implementation, I would vote to keep it.

[ephemient]

how is it easy? you have to re-create the Regex to change its i flag

[ephemient]

anyhow, without ignoreCase this is prefix.matchesAt(string, 0)

[nkiesel]

easy because we can use val pattern = if (ignoreCase) Regex(prefix.pattern, RegexOption.IGNORE_CASE) else prefix. Of course, we still have to wrap that in ^ and .*.

[ephemient]

you're losing other flags

[ephemient]

and if the regex is already case insensitive, shouldn't ignoreCase = false flip it?

[ephemient]

with matchesAt you don't need to wrap it in any anchors

[nkiesel]

loosing: yes. so val pattern = if (ignoreCase) Regex(prefix.pattern, prefix.options + RegexOption.IGNORE_CASE) else prefix. But yes, if the pattern already contains an ?i , then startsWith(pattern, false) would not do the right thing (assuming the in-pattern flags overwrite the outside-of-pattern flags). So perhaps makes more sense to drop the ignoreCase parameter for the `Regex`variant.

[nkiesel]

matchesAt works nicely for startsWith, but not so nice for endsWith.

[ephemient]

as far as I'm aware, there's no efficient way to implement endsWith on non-regular regexes (as found in basically everything aside from Google RE2)

[nkiesel]

yeah, I agree. But I'm not too concerned about performance here, and really am aiming for convenience and consistency. I can write fileName.contains(".kt") and filename.contains("Regex("""\.kt""") , and I can write fileName.endsWith(".kt"), so why not also fileName.endsWith(Regex("""\.kt""")) ?