open System.Text.RegularExpressions
let testString = "оди<tag1 par1>н дв</tag1>а<tag2 par2>три че </tag2>тыре д<li>в</li>а пять"
let whatToDelete = "два"
let tagPattern = new Regex("(?i)<.*?>")
let stringWithoutTags = tagPattern.Replace(testString, "")
let matchesToPairs matches = matches |> Seq.cast<Match> |> Seq.map(fun x -> (x.Index, x.Index + x.Value.Length - 1))
let forDelete = Regex.Matches(stringWithoutTags, whatToDelete) |> matchesToPairs 
let tagsPositions = tagPattern.Matches(testString) |> matchesToPairs
let inPairs pairs position = 
    pairs |> 
    Seq.exists (fun (startPosition, endPosition) -> (position >= startPosition) && (position <= endPosition))
let charactersWithPositions = testString |> Seq.mapi (fun pos char -> pos, char)
let previousTagsPositions position = tagsPositions |> Seq.takeWhile (fun x -> (fst x) < position)
let intervalLength (first, last) = last - first + 1
let positionWithoutTag position = position - (previousTagsPositions position |> Seq.sumBy intervalLength) 
let isDeleted position = inPairs forDelete (positionWithoutTag position)
let isTag position = inPairs tagsPositions position
let isGood position = (isTag position) || not (isDeleted position)
let resultSequence = 
    testString 
    |> Seq.mapi (fun pos char -> isGood pos, char) 
    |> Seq.where fst 
    |> Seq.map snd
    |> Seq.toArray
    

printf "%A" (new System.String(resultSequence))

open System.Text.RegularExpressions
let input = "оди<tag1 par1>н дв</tag1>а<tag2 par2>три че </tag2>тыре д<раз/><два>ва";
let pat = "два";
let tagPattern = "(<.*?>)*"
let replacePattern = pat |> Seq.map string |> String.concat tagPattern  
let joinCaptures (group:Group) = group.Captures |> Seq.cast<Capture> |> Seq.map (fun x -> x.Value) |> String.concat ""
let result = Regex.Replace(input, replacePattern, 
    fun (m:Match) -> 
        m.Groups 
        |> Seq.cast<Group> 
        |> Seq.skip 1 
        |> Seq.map joinCaptures 
        |> String.concat "" 
);

open System.Text.RegularExpressions
let input = "оди<tag1 par1>н дв</tag1>а<tag2 par2>три че </tag2>тыре д<раз/><два>ва";
let pat = "два";
let tagPattern = "((<.*?>)*)"
let replacePattern = pat |> Seq.map string |> String.concat tagPattern  
let replaceString = seq {1 .. 2 .. ((String.length pat) - 2) * 2 + 1 } |> Seq.map (sprintf "$%d")  |> String.concat ""
let result = Regex.Replace(input, replacePattern, replaceString)
printf "%s" result