VB NET regexp matching numerical substrings - regex

Question

I'm trying to make a vb function that takes as input a String and returns, if exist, the string made of numeric digits from the beginning until the first non numerical char, so:
123 -> 123
12f -> 12
12g34 -> 12
f12 -> ""
"" -> ""
I wrote a function that incrementally compares the result matching the regex, but it goes on even on non numeric characters...
This is the function:
Public Function ParseValoreVelocita(ByVal valoreRaw As String) As String
Dim result As New StringBuilder
Dim regexp As New Regex("^[0-9]+")
Dim tmp As New StringBuilder
Dim stringIndex As Integer = 0
Dim out As Boolean = False
While stringIndex < valoreRaw.Length AndAlso Not out
tmp.Append(valoreRaw.ElementAt(stringIndex))
If regexp.Match(tmp.ToString).Success Then
result.Append(valoreRaw.ElementAt(stringIndex))
stringIndex = stringIndex + 1
Else
out = True
End If
End While
Return result.ToString
End Function
The output always equals the input string, so there's something wrong and I can't get out of it...

Answer 1

Here's a LINQ solution that doesn't need regex and increases readability:
Dim startDigits = valoreRaw.TakeWhile(AddressOf Char.IsDigit)
Dim result As String = String.Concat(startDigits)

Answer 2

Try this instead. You need to use a capture group:
Public Function ParseValoreVelocita(ByVal valoreRaw As String) As String
Dim result As New StringBuilder
Dim regexp As New Regex("^([0-9]+)")
Dim tmp As New StringBuilder
Dim stringIndex As Integer = 0
Dim out As Boolean = False
While stringIndex < valoreRaw.Length AndAlso Not out
tmp.Append(valoreRaw.ElementAt(stringIndex))
If regexp.Match(tmp.ToString).Success Then
result.Append(regexp.Match(tmp.ToString).Groups(1).Value)
stringIndex = stringIndex + 1
Else
out = True
End If
End While
Return result.ToString
End Function
The expression:
Dim regexp As New Regex("^([0-9]+)")
and the result appending lines have been updated:
result.Append(regexp.Match(tmp.ToString).Groups(1).Value)

Answer 3

You have made your code very complex for a simple task.
Your loop keeps trying to build a longer string and it keeps checking if it is still working with digits, and if so keep appending results.
So and input string of "123x" would, if your code worked, produce a string of "112123" as output. In other words it matches the "1", then "12", then "123"and concatenates each before exiting after it finds the "x".
Here's what you should be doing:
Public Function ParseValoreVelocita(valoreRaw As String) As String
Dim regexp As New Regex("^([0-9]+)")
Dim match = regexp.Match(valoreRaw)
If match.Success Then
Return match.Groups(1).Captures(0).Value
Else
Return ""
End If
End Function
No loop and you let the regex do the work.