Find the unique positive integer whose square has the form 1_2_3_4_5_6_7_8_9_0, where each “_” is a single digit.



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open System.Text.RegularExpressions

let upperBound = 1389244398I // sqrt(1930000000000000000I)
let lowerBound = 1000000000I

//let rawPattern = "1_2_3_4_5_6_7_8_9_0".Replace("_", ".").ToString()
let pattern = Regex("1.2.3.4.5.6.7.8.9.0")

let matches n =
    let m = pattern.Match (string(n))
    m.Success

seq { lowerBound .. upperBound }
|> Seq.filter (fun n -> matches (n * n))
|> Seq.first



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