regex - How to use "MatchEvaluator" function to reduce coding?

Question

I'm trying to search the regular expression pattern and, if it matches, find whether the value of that pattern exists inside any tag of the form <sec id="sec123"> in a file. If it does, I want to replace it with result1. I think it can be done with the MatchEvaluator function, but I can't figure out how to apply it.

I'm new to VB.NET (and programming in general) and really don't know what to do. This is what I've tried so far:

Dim pattern As String="(?<=rid="sec)(\d+)(?=">)"
Dim r As Regex = New Regex(pattern)
Dim m As Match = r.Match(input)
If (m.Success) Then
    Dim x As String=" id=""sec"+ pattern +""""
    Dim r2 As Regex = New Regex(x)
    Dim m2 As Match = r2.Match(input)
    If (m2.Success) Then
        Dim tgPat AsString="<xref ref-type="section" rid=""sec + pattern +"">(w+) (d+)</xref>"
        Dim tgRep As String= "$1 $2"
        Dim tgReg As New Regex(tgPat)
        Dim result1 As String = tgReg.Replace(input, tgRep)
    Else
    EndIf
EndIf
Next

sample input:

<sec id="sec1">
<p>"You fig. 23 did?" I <xref ref-type="section" rid="sec12">section 12</a> asked, surprised.</p>
<p>"There are always better terms <xref ref-type="section" rid="sec6">section 6</a>, Richard!" my mom said sharply.</p>
<p>I <xref ref-type="section" rid="sec2">section 2</a> stood. I <xref ref-type="section" rid="sec2">section 2</a> had to hurry if I <xref ref-type="section" rid="sec1">section 1</a> was going to get to work on time.
<fig id="fig4">
<caption><p>I'm confused</p></caption>
</fig> 
</p>
<p>Turning to face her, I <xref ref-type="section" rid="sec2">section 2</a> walked backward. "I"ve seriously got to get ready. Why don"t we get together for lunch and talk more then?"</p>
<sec id="sec2">
<p>"You fig. 23 can"t be""</p>
<p>I <xref ref-type="section" rid="sec4">section 4</a> adored the Art Deco elegance of the Chrysler Building. I <xref ref-type="section" rid="sec2">section 2</a> could pinpoint my place on the island in relation to the posit table 9ion of the Empire State Building.</p>
<p>I <xref ref-type="section" rid="sec1">section 1</a> felt Gideon before I <xref ref-type="section" rid="sec1">section 1</a> saw him, my entire body humming wit table 9h awareness as he stepped out of the Bentley, which had pulled up behind the Benz.</p>
</sec>
</sec>

expected output:

<sec id="sec1">
<p>"You fig. 23 did?" I **section 12** asked, surprised.</p>
<p>"There are always better terms **section 6**, Richard!" my mom said sharply.</p>
<p>I <xref ref-type="section" rid="sec2">section 2</a> stood. I <xref ref-type="section" rid="sec2">section 2</a> had to hurry if I <xref ref-type="section" rid="sec1">section 1</a> was going to get to work on time.
<fig id="fig4">
<caption><p>I'm confused</p></caption>
</fig> 
</p>
<p>Turning to face her, I <xref ref-type="section" rid="sec2">section 2</a> walked backward. "I"ve seriously got to get ready. Why don"t we get together for lunch and talk more then?"</p>
<sec id="sec2">
<p>"You fig. 23 can"t be""</p>
<p>I **section 4** adored the Art Deco elegance of the Chrysler Building. I <xref ref-type="section" rid="sec2">section 2</a> could pinpoint my place on the island in relation to the posit table 9ion of the Empire State Building.</p>
<p>I <xref ref-type="section" rid="sec1">section 1</a> felt Gideon before I <xref ref-type="section" rid="sec1">section 1</a> saw him, my entire body humming wit table 9h awareness as he stepped out of the Bentley, which had pulled up behind the Benz.</p>
</sec>
</sec>

Answer 1

I've replaced my initial solution with one which uses XElement as your data isn't as straightforward as I initially thought.

    Dim input = XElement.Parse(data)
    Dim sections = input.Descendants("sec").ToDictionary(Function(s) s.@id, Function(s) s)
    Dim xrefs = input.Descendants("xref").ToLookup(Function(s) s.@rid, Function(s) s)

    For Each group In xrefs
        Dim section As XElement
        If sections.TryGetValue(group.Key, section) Then
            For Each xref In group
                xref.ReplaceWith(section)
            Next
        End If
    Next
    Dim output = input.ToString()

I think this does what you're after although I don't trust your data as Section2 seems to be recursive. Have a try and see what you think anyway. Steps:

1. Parse the xml
2. Extract the sections and key them by id in a dictionary
3. Extract the xrefs (multiple with same key) add them by id to a Lookup
4. For each xref check if it has a section. If it does then replace it.
5. Extract the result.