Excel separate a line of text

Using TEXTSPLIT to create an array and FILTERing and TEXTJOIN:

=LET(_z,TEXTSPLIT(A1,"_","|"),_u,UNIQUE(TAKE(_z,,1)),TRANSPOSE(BYROW(_u,LAMBDA(_y,TEXTJOIN("|",,FILTER(TAKE(_z,,1)&"_"&TAKE(_z,,-1),TAKE(_z,,1)=_y))))))

If I have understood correctly, then the following formula should do the task you are aiming for, note that the formulas exclusively works with MS365 per the tags posted.

• Method One:

=LET(
     a, TEXTSPLIT(B2,,"|"),
     b, TEXTBEFORE(a,"_"),
     DROP(GROUPBY(b,a,LAMBDA(x,TEXTJOIN("|",1,x)),,0),,1))

• Method Two:

=LET(
     a, TEXTSPLIT(B2,,"|"),
     b, TEXTBEFORE(a,"_"),
     c, UNIQUE(b),
     MAP(c,LAMBDA(x,TEXTJOIN("|",1,FILTER(a,b=x)))))

Or, With ETA :

=LET(
     a, TEXTSPLIT(B2,"_","|"),
     b, TAKE,
     DROP(GROUPBY(b(a,,1),b(a,,1)&"_"&b(a,,-1),LAMBDA(x,TEXTJOIN("|",,x)),,0),,1))

• For multiple rows of data then using REDUCE():

=DROP(REDUCE("",B2:B3,LAMBDA(x,y,
 LET(z, TEXTSPLIT(y,"_","|"), 
     w, TAKE, VSTACK(x,
     DROP(GROUPBY(w(z,,1),w(z,,1)&"_"&w(z,,-1),
     LAMBDA(v,TEXTJOIN("|",1,v)),,0),,1))))),1)

The formula in A4
no is the numbers to check in the filenames
tsep is the array of the filenames

=LET(sep,TEXTSPLIT(A1,"imagefile"),
no,DROP(MID(sep,1,SEARCH("_",sep)-1),,1),
tsep,TEXTSPLIT(A1,"|"),
fg,BYROW(SEQUENCE(MAX(VALUE(no))),LAMBDA(a,TEXTJOIN("|",TRUE,IF(a=VALUE(no),tsep,"")))),
fg)

Assuming the input text is in cell A1, the output will begin in cell A3.

Microsoft documentation:

Dictionary object

Range.Resize property (Excel)

Split function

Option Explicit
Sub Demo()
    Const SEP = "|"
    Dim objDic As Object, rngData As Range
    Dim i As Long, sKey As String
    Set objDic = CreateObject("scripting.dictionary")
    Dim sTxt As String: sTxt = Range("A1")
    Dim aTxt: aTxt = Split(sTxt, SEP)
    For i = LBound(aTxt) To UBound(aTxt)
        sKey = Split(aTxt(i), "_")(0)
        If objDic.exists(sKey) Then
            objDic(sKey) = objDic(sKey) & SEP & aTxt(i)
        Else
            objDic(sKey) = aTxt(i)
        End If
    Next
    If objDic.Count > 0 Then
        Range("A3").Resize(objDic.Count, 1) = Application.Transpose(objDic.items)
    End If
End Sub

Formula in A3:

=TOCOL(REGEXEXTRACT(A1,"\b([^_|]+)(_(?1)+\b)(?:\|\1(?2))*",1))

Tricky Array matching with VBA vers. 2010

As one option suggested by OP and also following Scott Craner's advice I wanted to demonstrate some tricky & possibly instructive methods applied upon arrays by means of 2010 VBA only:

• Tokenizing and regrouping of Array elements by subsequent Splitting (→ 1.1 Main function getElems())

• Split combined with negative filtering to isolate base names as well as ..

• Match comparing array elements with itself instead of iterative search of single elements within a given array (→ 1.2 Helper function FirstPos())

Furthermore his approach allows different filename prefixes and provides also for empty or error inputs.

I don't intend, however to show a better or even shorter approach than @taller's ingeniously short & simple solution (via a dictionary object).

1.1 Main function getElems()

Function getElems(ByVal s As Variant)
'Purp: Group file names by splitting
'Site: https://stackoverflow.com/questions/79228137/excel-separate-a-line-of-text
'Auth: https://stackoverflow.com/users/6460297/t-m
'a) tokenize elements
    Dim elems: elems = Split(s, "|")
'b) find & mark unique start file
    Dim pos
    For Each pos In FirstPos(s)     ' << 
        If IsNumeric(pos) Then s = Replace(s, "|" & elems(pos - 1), "$" & elems(pos - 1))
        Debug.Print pos
    Next
'c) rebuild elements including subordinated file names
    getElems = Split(s, "$")
End Function

1.2 Helper function FirstPos()

Function FirstPos(ByVal s As Variant)
'a) get base names (file name before "_")
    Dim tmp: tmp = Replace(s, "_", "|_")
    Dim arr: arr = Filter(Split(tmp, "|"), "_", False)
'b) return matched base name positions
'   via tricky MATCH() function comparing array elements with itself
    FirstPos = Application.Match(arr, arr, 0)
End Function

2. Example call

Note ad) A) Get input: Outcomment the Test assignment to the input string s = ... in the upper code line if you want to refer to a range value and activate the outcommented alternative block below instead.

Sub DemoTM()
    Const ERROR = "ERROR!": Const NOTHG = "EMPTY!"    ' at least 1 character (or 1 blank!)
'A) get input
    Dim s
    s = "imagefile1_1.jpg|imagefile1_2.jpg|imagefile2_1.jpg|imagefile3_2.jpg|imagefile3_1.jpg"
    ' 'alternatively:
    ' s = Sheet1.Range("A1").Value2 ' << change worksheet as needed
'B) treat exceptions
    If IsError(s) Then s = ERROR
    If s = "" Then s = NOTHG            ' simulate blank instead of empty element
'C) get regrouped elements
    Dim elems: elems = getElems(s)      ' << see function getElems()
'D) display output in Immediate Window and/or target range
    Debug.Print Join(elems, vbNewLine)
    Sheet1.Range("D3").Resize(UBound(elems) + 1, 1) = Application.Transpose(elems)  ' << change sheet as needed
End Sub

Output in VB Editor's Immediate window:

imagefile1_1.jpg|imagefile1_2.jpg
imagefile2_1.jpg
imagefile3_2.jpg|imagefile3_1.jpg