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RegularExpression

This is a class module that performs regular expression searches in a string.

Inputs
Use the Init method to initialize to a specific regular expression (which will be precompiled), then use Match to check if a string contains such a substring.

Assumes
a) Put this code in a new .CLS file (NOT in a class module). 'b) The syntax for range (e.g. [a-z]) is the same as for operator Like. 'c) Not calling Init, or passing an empty pattern, will result in an "Illegal function call" error.

API Declarations



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VERSION 1.0 CLASS
BEGIN
 MultiUse = -1 'True
END
Attribute VB_Name = "RegularExpression"
Attribute VB_GlobalNameSpace = False
Attribute VB_Creatable = True
Attribute VB_PredeclaredId = False
Attribute VB_Exposed = False
Option Explicit
'PRIVATE
'? = edOptional; + = edMulti; * = edOptional or edMulti
Private Enum RegExpStateTypes
  edOptional = 65536
  edMulti = 131072
  edModifierMask = edOptional Or edMulti
  
  edCharacter = 0
  edBracketed = 262144    'for example, [a-z]
  edAny = 524288
End Enum
Private Type StateStack
  State As Long
  Posi As Long
  MinPosi As Long
End Type
Private mStack() As StateStack
Private mCompiled() As Long
Private mNStates As Long
Private mPattern As String
Private mAnchorBeginning As Boolean
Private mAnchorEnd As Boolean
Private mMinLength As Long

Private Sub AddState(ByVal Flags As Long, ByVal CharOrPosi As Long)
If mNStates = UBound(mCompiled) Then
  ReDim Preserve mCompiled(1 To mNStates + 10) As Long
End If
mNStates = mNStates + 1
mCompiled(mNStates) = CharOrPosi Or Flags
End Sub
Public Sub Init(RegExp As String)
Dim StackSize As Long, Posi As Long, EndPosi As Long
'Initialize member variables
mPattern = RegExp
mNStates = 0
mMinLength = 0
ReDim mCompiled(1 To 10) As Long
Posi = 1
EndPosi = Len(RegExp)
If Left(mPattern, 1) = "^" Then
  Posi = Posi + 1
  mAnchorBeginning = True
End If
If Right(mPattern, 1) = "$" And Right(mPattern, 2) <> "\$" Then
  EndPosi = EndPosi - 1
  mAnchorEnd = True
End If
Do Until Posi > EndPosi
  Select Case Mid$(mPattern, Posi, 1)
    Case "."
      AddState edAny, 0
      Posi = Posi + 1
    Case "\"
      AddState edCharacter, Asc(Mid$(mPattern, Posi + 1, 1))
      Posi = Posi + 2
    Case "["
      AddState edBracketed, Posi
      Posi = RangeParse(Posi)
      If Posi = -1 Then Err.Raise 5
    Case Else
      AddState edCharacter, Asc(Mid$(mPattern, Posi, 1))
      Posi = Posi + 1
  End Select
  
  'check for modifiers (?, +, *)
  Select Case Mid$(mPattern, Posi, 1)
    Case "?"
      mCompiled(mNStates) = mCompiled(mNStates) Or edOptional
      StackSize = StackSize + 1
      Posi = Posi + 1
    Case "+"
      mCompiled(mNStates) = mCompiled(mNStates) Or edMulti
      StackSize = StackSize + 1
      Posi = Posi + 1
      mMinLength = mMinLength + 1
    Case "*"
      mCompiled(mNStates) = mCompiled(mNStates) Or edMulti Or edOptional
      StackSize = StackSize + 1
      Posi = Posi + 1
    Case Else
      mMinLength = mMinLength + 1
  End Select
Loop
'Minimize wasted memory by dimensioning exact arrays
ReDim Preserve mCompiled(1 To mNStates) As Long
ReDim mStack(1 To StackSize) As StateStack
End Sub
Public Function Match(ByRef FromX As Long, ByRef ToX As Long, Text As String) As Boolean
Dim Match As Boolean
Dim CurState As Long
Dim State As Long
Dim SP As Long
Dim LenText As Long
If mNStates = 0 Then Err.Raise 5
LenText = Len(Text)
For FromX = FromX To IIf(mAnchorBeginning, 1, LenText - mMinLength)
  ToX = FromX
  State = 1
  SP = 0
  Do
    If State > mNStates Then
      If (Not mAnchorEnd) Or (ToX > LenText) Then
        'ToX is pointing the first character PAST the matched string
        ToX = ToX - 1
        MatchRight = True
        Exit Function
      End If
    End If
    GoSub MatchState
    If Match Then
      If CurState And edModifierMask Then
        'create a new item in the backtrack stack
        SP = SP + 1
        mStack(SP).MinPosi = IIf(CurState And edOptional, ToX, ToX + 1)
        If (CurState And (edAny Or edMulti)) = (edAny Or edMulti) Then
          'When matching .* and .+, we don't need to check the whole string
          ToX = LenText + 1
        ElseIf CurState And edMulti Then
          '+ or *, try to get as far as possible
          Do
            ToX = ToX + 1
            GoSub MatchState
          Loop Until Not Match
        Else
          '?, you only have to look one character ahead
          ToX = ToX + 1
        End If
        State = State + 1
        mStack(SP).Posi = ToX
        mStack(SP).State = State
      Else
        'no +, *, nor ?, just advance to the next state
        ToX = ToX + 1
        State = State + 1
      End If
    ElseIf CurState And edOptional Then
      'not matched, but it was optional... no problem
      State = State + 1
    Else
      'backtrack - find the next usable item in the stack
      For SP = SP To 1 Step -1
        If mStack(SP).Posi > mStack(SP).MinPosi Then Exit For
      Next SP
      If SP = 0 Then Exit Do
      mStack(SP).Posi = mStack(SP).Posi - 1
      ToX = mStack(SP).Posi
      State = mStack(SP).State
    End If
  Loop
Next FromX
Exit Function
MatchState:
  CurState = mCompiled(State)
  If ToX > LenText Then
    Match = False
  ElseIf CurState And edAny Then
    Match = True
  ElseIf CurState And edBracketed Then
    Match = RangeMatch(CurState And 65535, Mid$(Text, ToX, 1))
  Else
    Match = (CurState And 65535) = Asc(Mid$(Text, ToX, 1))
  End If
  Return
End Function
Private Function RangeMatch(Posi As Long, ch As String) As Boolean
RangeMatch = ch Like Mid$(mPattern, Posi, InStr(Posi, mPattern, "]") - Posi + 1)
End Function

'Return the end of the range (e.g. [a-z]) starting at position Posi.
'Return -1 if the regular expression is not well formed.
Private Function RangeParse(Posi As Long) As Long
Dim EndPosi As Long
EndPosi = InStr(Posi, mPattern, "]")
'Try using operator Like and check if an error occurs
On Error Resume Next
If "a" Like Mid(mPattern, Posi, EndPosi - Posi + 1) Then:
If Err Then
  RangeParse = -1
  Err.Clear
Else
  RangeParse = EndPosi + 1
End If
End Function