Question

In the following example code, I filter a list of strings on a regular expression, knowing that there can only be a single entry that will match that string. I then use the same match string to get 2 grouped values out of the single remaining value.

let input = ["aaaa bbbb";"aaabbbb";"cccc$$$$";"dddddda";" "] 

let ValuesOfAB (input: string list) = 
    let matchString = "(?<a>\w+)\s(?<b>\w+)"

    let value = input |> List.filter (fun line -> Regex.Matches(line, matchString).Count <> 0) 
                      |> List.head
    (Regex.Matches(value, matchString).[0].Groups.["a"].Value, Regex.Matches(value, matchString).[0].Groups.["b"].Value)

let a = ValuesOfAB input

Is there a better way where I don’t have to use Regex.Matches on the same string again for a second time to get the values I wish to return?

Answer 1

Use List.pick:

let input = ["aaaa bbbb";"aaabbbb";"cccc$$$$";"dddddda";" "] 

let valuesOfAB (input: string list) = 
    let matchString = "(?<a>\w+)\s(?<b>\w+)"
    let v = input |> List.pick (fun line -> let m = Regex.Match(line, matchString)
                                            if m.Success then Some m else None)
    v.Groups.["a"].Value, v.Groups.["b"].Value

let a = valuesOfAB input

Explanation:

You would like to match the first string in the list and return Match object in order that you don’t have to run Regex again. List.pick fits the task quite well.

With each string, you need to match at least once so Regex.Match and Match.Success is enough for the purpose.