Update snippet 'Parse and replace string with templating characters'

Title

Description

Source code

let parseAndReplaceTemplate
    (startPosition: int)
    (startTag: string)
    (endTag: string)
    (replacements: Map<string, string>)
    (template: string)
    : string =
    // sanity checks (do once)
    if System.String.IsNullOrWhiteSpace startTag
       || System.String.IsNullOrWhiteSpace endTag
       || System.String.IsNullOrWhiteSpace template
       || replacements.IsEmpty
       || startPosition > (template.Length - 1) then
        template // return unaltered
    else
        let regexStr = startTag + "(.*?)" + endTag
        let regex = System.Text.RegularExpressions.Regex (regexStr)

let rec replace (input: string) =
            let regexMatch = regex.Match input

if regexMatch.Success then
                let key = regexMatch.Groups.[1].Value
                let value = replacements |> Map.tryFind key

match value with
                | Some v ->
                    let before = input.Substring (0, regexMatch.Index)
                    let after = input.Substring (regexMatch.Index + regexMatch.Length)
                    before + v + replace after
                | None -> replace input
            else
                input

let skippedText = template.Substring (0, startPosition)
        let output = replace (template.Substring startPosition)

skippedText + output