CodeSOD: Philegex

Last week, I was doing some graphics programming without a graphics card. It was low resolution, so I went ahead and re-implemented a few key methods from the Open GL Shader Language in a fashion which was compatible with NumPy arrays. Lucky for me, I was able to draw off many years of experience, I understood both technologies, and they both have excellent documentation which made it easy. After dozens of lines of code, I was able to whip up some pretty flexible image generator functions. I knew the tools I needed, I understood how they worked, and while I was reinventing a wheel, I had a very specific reason.

Philemon Eichin sends us some code from a point in his career where none of these things were true.

Philemon was building a changelog editor. As such, he wanted an easy, flexible way to identify patterns in the text. Philemon knew that there was something that could do that job, but he didn't know what it was called or how it was supposed to work. So, like all good programmers, Philemon went ahead and coded up what he needed- he invented his own regular expression language, and built his own parser for it.

Thus was born Philegex. Philemon knew that regexes involved slashes, so in his language you needed to put a slash in front of every character you wanted to match exactly. He knew that it involved question marks, so he used the question mark as a wildcard which could match any character. That left the '|" character to be optional.

So, for example: /P/H/I/L/E/G/E/X|??? would match "PHILEGEX!!!" or "PHILEGEWTF". A date could be described as: nnnn/.nn/.nn. (YYYY.MM.DD or YYYY.DD.MM)

Living on his own isolated island without access to the Internet to attempt to google up "How to match patterns in text", Philemon invented his own language for describing parts of a regular expression. This will be useful to interpret the code below.

Philegex	Regex
Maskable	Matches
p1	Pattern / Regex
Block(s)	Token(s)
CT	CharType
SplitLine	ParseRegex
CC	currentChar
auf_zu	openParenthesis
Chars	CharClassification

With the preamble out of the way, enjoy Philemon's approach to regular expressions, implemented elegantly in VB.Net.

Public Class Textmarker Const Datum As String = "nn/.nn/.nnnn" Private Structure Blocks Dim Type As Chars Dim Multi As Boolean Dim Mode As Char_Mode Dim Subblocks() As Blocks Dim passed As Boolean Dim _Optional As Boolean End Structure Public Shared Function IsMaskable(p1 As String, Content As String) As Boolean Dim ID As Integer = 0 Dim p2 As Chars Dim _Blocks() As Blocks = SplitLine(p1) For i As Integer = 0 To Content.Length - 1 p2 = GetCT(Content(i))START_CASE: '#If CONFIG = "Debug" Then ' If ID = 2 Then ' Stop ' End If '#End If If ID > _Blocks.Length - 1 Then Return False End If Select Case _Blocks(ID).Mode Case Char_Mode._Char If p2.Char_V = _Blocks(ID).Type.Char_V Then _Blocks(ID).passed = True If Not _Blocks(ID).Multi = True Then ID += 1 Exit Select Else If _Blocks(ID).passed = True And _Blocks(ID).Multi = True Then ID += 1 GoTo START_CASE Else If Not _Blocks(ID)._Optional Then Return False End If End If Case Char_Mode.Type If _Blocks(ID).Type.Type = Chartypes.any Then _Blocks(ID).passed = True If Not _Blocks(ID).Multi = True Then ID += 1 Exit Select Else If p2.Type = _Blocks(ID).Type.Type Then _Blocks(ID).passed = True If Not _Blocks(ID).Multi = True Then ID += 1 Exit Select Else If _Blocks(ID).passed = True And _Blocks(ID).Multi = True Then ID += 1 GoTo START_CASE Else If _Blocks(ID)._Optional Then ID += 1 _Blocks(ID - 1).passed = True Else Return False End If End If End If End If End Select Next For i = ID To _Blocks.Length - 1 If _Blocks(ID)._Optional = True Then _Blocks(ID).passed = True Else Exit For End If Next If _Blocks(_Blocks.Length - 1).passed Then Return True Else Return False End If End Function Private Shared Function GetCT(Char_ As String) As Chars If "0123456789".Contains(Char_) Then Return New Chars(Char_, 2) If "qwertzuiopi1/4asdfghjkliiyxcvbnmi".Contains((Char.ToLower(Char_))) Then Return New Chars(Char_, 1) Return New Chars(Char_, 4) End Function Private Shared Function SplitLine(ByVal Line As String) As Blocks() Dim ret(0) As Blocks Dim retID As Integer = -1 Dim CC As Char For i = 0 To Line.Length - 1 CC = Line(i) Select Case CC Case "(" ReDim Preserve ret(retID + 1) retID += 1 Dim ii As Integer = i + 1 Dim auf_zu As Integer = 1 Do Select Case Line(ii) Case "(" auf_zu += 1 Case ")" auf_zu -= 1 Case "/" ii += 1 End Select ii += 1 Loop Until auf_zu = 0 ret(retID).Subblocks = SplitLine(Line.Substring(i + 1, ii - 1)) ret(retID).Mode = Char_Mode.subitems ret(retID).passed = False Case "*" ret(retID).Multi = True ret(retID).passed = False Case "|" ret(retID)._Optional = True Case "/" ReDim Preserve ret(retID + 1) retID += 1 ret(retID).Mode = Char_Mode._Char ret(retID).Type = New Chars(Line(i + 1), Chartypes.other) i += 1 ret(retID).passed = False Case Else ReDim Preserve ret(retID + 1) retID += 1 ret(retID).Mode = Char_Mode.Type ret(retID).Type = New Chars(Line(i), TocType(CC)) ret(retID).passed = False End Select Next Return ret End Function Private Shared Function TocType(p1 As Char) As Chartypes Select Case p1 Case "c" Return Chartypes._Char Case "n" Return Chartypes.Number Case "?" Return Chartypes.any Case Else Return Chartypes.other End Select End Function Public Enum Char_Mode As Integer Type = 1 _Char = 2 subitems = 3 End Enum Public Enum Chartypes As Integer _Char = 1 Number = 2 other = 4 any End Enum Structure Chars Dim Char_V As Char Dim Type As Chartypes Sub New(Char_ As Char, typ As Chartypes) Char_V = Char_ Type = typ End Sub End StructureEnd Class

I'll say this: building a finite state machine, which is what the core of a regex engine is, is perhaps the only case where using a GoTo could be considered acceptable. So this code has that going for it. Philemon was kind enough to share this code with us, so we knew he knows it's bad.

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