Implementing String.Format() in VB6

I couldn't find one anywhere, so I made my own:

Public PADDING_CHAR As StringPublic Function StringFormat(format_string As String, ParamArray values()) As String'VB6 implementation of .net String.Format(), slightly customized.'Tested with Office 2010 VBA (x64)        Dim return_value As String        Dim values_count As Integer        'some error-handling constants:        Const ERR_FORMAT_EXCEPTION As Long = vbObjectError Or 9001        Const ERR_ARGUMENT_NULL_EXCEPTION As Long = vbObjectError Or 9002        Const ERR_SOURCE As String = "StringFormat"        Const ERR_MSG_INVALID_FORMAT_STRING As String = "Invalid format string."        Const ERR_MSG_FORMAT_EXCEPTION As String = "The number indicating an argument to format is less than zero, or greater than or equal to the length of the args array."        'use SPACE as default padding character        If PADDING_CHAR = vbNullString Then PADDING_CHAR = Chr$(32)        'figure out number of passed values:        values_count = UBound(values) + 1        Dim regex As RegExp        Dim matches As MatchCollection        Dim thisMatch As Match        Dim thisString As String        Dim thisFormat As String        'when format_string starts with "@", escapes are not replaced         '(string is treated as a literal string with placeholders)                Dim useLiteral As Boolean         Dim escapeHex As Boolean 'indicates whether HEX specifier "0x" is to be escaped or not        'validate string_format:        Set regex = New RegExp        regex.Pattern = "{({{)*(\w+)(,-?\d+)?(:[^}]+)?}(}})*"        regex.IgnoreCase = True        regex.Global = True        Set matches = regex.Execute(format_string)        'determine if values_count matches number of unique regex matches:        Dim uniqueCount As Integer        Dim tmpCSV As String        For Each thisMatch In matches            If Not StringContains(tmpCSV, thisMatch.SubMatches(1)) Then                uniqueCount = uniqueCount + 1                tmpCSV = tmpCSV & thisMatch.SubMatches(1) & ","            End If        Next        'unique indices count must match values_count:        If matches.Count > 0 And uniqueCount <> values_count Then _            Err.Raise ERR_FORMAT_EXCEPTION, _            ERR_SOURCE, ERR_MSG_FORMAT_EXCEPTION        useLiteral = StringStartsWith("@", format_string)        'remove the "@" literal specifier        If useLiteral Then format_string = Right(format_string, Len(format_string) - 1)        If Not useLiteral And StringContains(format_string, "\\") Then _            format_string = Replace(format_string, "\\", Chr$(27))        If StringContains(format_string, "\\") Then _            format_string = Replace(format_string, "\\", Chr$(27))        If matches.Count = 0 And format_string <> vbNullString And UBound(values) = -1 Then        'only format_string was specified: skip to checking escape sequences:            return_value = format_string            GoTo checkEscapes        ElseIf UBound(values) = -1 And matches.Count > 0 Then            Err.Raise ERR_ARGUMENT_NULL_EXCEPTION, _                ERR_SOURCE, ERR_MSG_FORMAT_EXCEPTION        End If        return_value = format_string        'dissect format_string:        Dim i As Integer, v As String, p As String 'i: iterator; v: value; p: placeholder        Dim alignmentGroup As String, alignmentSpecifier As String        Dim formattedValue As String, alignmentPadding As Integer        'iterate regex matches (each match is a placeholder):        For i = 0 To matches.Count - 1            'get the placeholder specified index:            Set thisMatch = matches(i)            p = thisMatch.SubMatches(1)            'if specified index (0-based) > uniqueCount (1-based), something's wrong:            If p > uniqueCount - 1 Then _                Err.Raise ERR_FORMAT_EXCEPTION, _                ERR_SOURCE, ERR_MSG_FORMAT_EXCEPTION            v = values(p)            'get the alignment specifier if it is specified:            alignmentGroup = thisMatch.SubMatches(2)            If alignmentGroup <> vbNullString Then _                alignmentSpecifier = Right$(alignmentGroup, LenB(alignmentGroup) / 2 - 1)            'get the format specifier if it is specified:            thisString = thisMatch.Value            If StringContains(thisString, ":") Then                Dim formatGroup As String, precisionSpecifier As Integer                Dim formatSpecifier As String, precisionString As String                'get the string between ":" and "}":                formatGroup = Mid$(thisString, InStr(1, thisString, ":") + 1, (LenB(thisString) / 2) - 2)                formatGroup = Left$(formatGroup, LenB(formatGroup) / 2 - 1)                precisionString = Right$(formatGroup, LenB(formatGroup) / 2 - 1)                formatSpecifier = Mid$(thisString, InStr(1, thisString, ":") + 1, 1)                'applicable formatting depends on the type of the value (yes, GOTO!!):                If TypeName(values(p)) = "Date" Then GoTo DateTimeFormatSpecifiers                If v = vbNullString Then GoTo ApplyStringFormatNumberFormatSpecifiers:                If precisionString <> vbNullString And Not IsNumeric(precisionString) Then _                    Err.Raise ERR_FORMAT_EXCEPTION, _                        ERR_SOURCE, ERR_MSG_INVALID_FORMAT_STRING                If precisionString = vbNullString Then precisionString = 0                Select Case formatSpecifier                    Case "C", "c" 'CURRENCY format, formats string as currency.                    'Precision specifier determines number of decimal digits.                    'This implementation ignores regional settings                    '(hard-coded group separator, decimal separator and currency sign).                    precisionSpecifier = CInt(precisionString)                    thisFormat = "#,##0.$"                    If LenB(formatGroup) > 2 And precisionSpecifier > 0 Then                         'if a non-zero precision is specified...                        thisFormat = _                        Replace$(thisFormat, ".", "." & String$(precisionString, Chr$(48)))                    End If                    Case "D", "d" 'DECIMAL format, formats string as integer number.                    'Precision specifier determines number of digits in returned string.                    precisionSpecifier = CInt(precisionString)                    thisFormat = "0"                    thisFormat = Right$(String$(precisionSpecifier, "0") & thisFormat, _                        IIf(precisionSpecifier = 0, Len(thisFormat), precisionSpecifier))                    Case "E", "e" 'EXPONENTIAL NOTATION format (aka "Scientific Notation")                    'Precision specifier determines number of decimals in returned string.                    'This implementation ignores regional settings'                    '(hard-coded decimal separator).                    precisionSpecifier = CInt(precisionString)                    thisFormat = "0.00000#" & formatSpecifier & "-#" 'defaults to 6 decimals                    If LenB(formatGroup) > 2 And precisionSpecifier > 0 Then                        'if a non-zero precision is specified...                        thisFormat = "0." & String$(precisionSpecifier - 1, Chr$(48)) & "#" & formatSpecifier & "-#"                    ElseIf LenB(formatGroup) > 2 And precisionSpecifier = 0 Then                        Err.Raise ERR_FORMAT_EXCEPTION, _                            ERR_SOURCE, ERR_MSG_INVALID_FORMAT_STRING                    End If                    Case "F", "f" 'FIXED-POINT format                    'Precision specifier determines number of decimals in returned string.                    'This implementation ignores regional settings'                    '(hard-coded decimal separator).                    precisionSpecifier = CInt(precisionString)                    thisFormat = "0"                    If LenB(formatGroup) > 2 And precisionSpecifier > 0 Then                        'if a non-zero precision is specified...                        thisFormat = (thisFormat & ".") & String$(precisionSpecifier, Chr$(48))                    Else                        'no precision specified - default to 2 decimals:                        thisFormat = "0.00"                    End If                    Case "G", "g" 'GENERAL format (recursive)                    'returns the shortest of either FIXED-POINT or SCIENTIFIC formats in case of a Double.                    'returns DECIMAL format in case of a Integer or Long.                    Dim eNotation As String, ePower As Integer, specifier As String                    precisionSpecifier = IIf(CInt(precisionString) > 0, CInt(precisionString), _                        IIf(StringContains(v, "."), Len(v) - InStr(1, v, "."), 0))                    'track character case of formatSpecifier:                    specifier = IIf(formatSpecifier = "G", "D", "d")                    If TypeName(values(p)) = "Integer" Or TypeName(values(p)) = "Long" Then                        'Integer types: use {0:D} (recursive call):                        formattedValue = StringFormat("{0:" & specifier & "}", values(p))                    ElseIf TypeName(values(p)) = "Double" Then                        'Non-integer types: use {0:E}                        specifier = IIf(formatSpecifier = "G", "E", "e")                        'evaluate the exponential notation value (recursive call):                        eNotation = StringFormat("{0:" & specifier & "}", v)                        'get the power of eNotation:                        ePower = Mid$(eNotation, InStr(1, UCase$(eNotation), "E-") + 1, Len(eNotation) - InStr(1, UCase$(eNotation), "E-"))                        If ePower > -5 And Abs(ePower) < precisionSpecifier Then                            'use {0:F} when ePower > -5 and abs(ePower) < precisionSpecifier:                            'evaluate the floating-point value (recursive call):                             specifier = IIf(formatSpecifier = "G", "F", "f")                             formattedValue = StringFormat("{0:" & formatSpecifier & _                                 IIf(precisionSpecifier <> 0, precisionString, vbNullString) & "}", values(p))                        Else                            'fallback to {0:E} if previous rule didn't apply:                            formattedValue = eNotation                        End If                    End If                    GoTo AlignFormattedValue 'Skip the "ApplyStringFormat" step, it's applied already.                    Case "N", "n" 'NUMERIC format, formats string as an integer or decimal number.                    'Precision specifier determines number of decimal digits.                    'This implementation ignores regional settings'                    '(hard-coded group and decimal separators).                    precisionSpecifier = CInt(precisionString)                    If LenB(formatGroup) > 2 And precisionSpecifier > 0 Then                        'if a non-zero precision is specified...                        thisFormat = "#,##0"                        thisFormat = (thisFormat & ".") & String$(precisionSpecifier, Chr$(48))                    Else 'only the "D" is specified                        thisFormat = "#,##0"                    End If                    Case "P", "p" 'PERCENT format. Formats string as a percentage.                    'Value is multiplied by 100 and displayed with a percent symbol.                    'Precision specifier determines number of decimal digits.                    thisFormat = "#,##0%"                    precisionSpecifier = CInt(precisionString)                    If LenB(formatGroup) > 2 And precisionSpecifier > 0 Then                        'if a non-zero precision is specified...                        thisFormat = "#,##0"                        thisFormat = (thisFormat & ".") & String$(precisionSpecifier, Chr$(48))                    Else 'only the "P" is specified                        thisFormat = "#,##0"                    End If                    'Append the percentage sign to the format string:                    thisFormat = thisFormat & "%"                    Case "R", "r" 'ROUND-TRIP format (a string that can round-trip to an identical number)                    'example: ?StringFormat("{0:R}", 0.0000000001141596325677345362656)                    '         ...returns "0.000000000114159632567735"                    'convert value to a Double (chop off overflow digits):                    v = CDbl(v)                    Case "X", "x" 'HEX format. Formats a string as a Hexadecimal value.                    'Precision specifier determines number of total digits.                    'Returned string is prefixed with "&H" to specify Hex.                    v = Hex(v)                    precisionSpecifier = CInt(precisionString)                    If LenB(precisionString) > 0 Then 'precision here stands for left padding                        v = Right$(String$(precisionSpecifier, "0") & v, IIf(precisionSpecifier = 0, Len(v), precisionSpecifier))                    End If                    'add C# hex specifier, apply specified casing:                    '(VB6 hex specifier would cause Format() to reverse the formatting):                    v = "0x" & IIf(formatSpecifier = "X", UCase$(v), LCase$(v))                    Case Else                        If IsNumeric(formatSpecifier) And val(formatGroup) = 0 Then                            formatSpecifier = formatGroup                            v = Format(v, formatGroup)                        Else                            Err.Raise ERR_FORMAT_EXCEPTION, _                                ERR_SOURCE, ERR_MSG_INVALID_FORMAT_STRING                        End If                End Select                GoTo ApplyStringFormatDateTimeFormatSpecifiers:                Select Case formatSpecifier                    Case "c", "C" 'CUSTOM date/time format                    'let VB Format() parse precision specifier as is:                        thisFormat = precisionString                    Case "d" 'SHORT DATE format                        thisFormat = "ddddd"                     Case "D" 'LONG DATE format                        thisFormat = "dddddd"                    Case "f" 'FULL DATE format (short)                        thisFormat = "dddddd h:mm AM/PM"                    Case "F" 'FULL DATE format (long)                        thisFormat = "dddddd ttttt"                    Case "g"                        thisFormat = "ddddd hh:mm AM/PM"                    Case "G"                        thisFormat = "ddddd ttttt"                    Case "s" 'SORTABLE DATETIME format                        thisFormat = "yyyy-mm-ddThh:mm:ss"                    Case "t" 'SHORT TIME format                        thisFormat = "hh:mm AM/PM"                    Case "T" 'LONG TIME format                        thisFormat = "ttttt"                    Case Else                        Err.Raise ERR_FORMAT_EXCEPTION, _                            ERR_SOURCE, ERR_MSG_INVALID_FORMAT_STRING                End Select                GoTo ApplyStringFormat            End IfApplyStringFormat:            'apply computed format string:            formattedValue = Format(v, thisFormat)AlignFormattedValue:            'apply specified alignment specifier:            If alignmentSpecifier <> vbNullString Then                alignmentPadding = Abs(CInt(alignmentSpecifier))                If CInt(alignmentSpecifier) < 0 Then                    'negative: left-justified alignment                    If alignmentPadding - Len(formattedValue) > 0 Then _                        formattedValue = formattedValue & _                            String$(alignmentPadding - Len(formattedValue), PADDING_CHAR)                Else                    'positive: right-justified alignment                    If alignmentPadding - Len(formattedValue) > 0 Then _                        formattedValue = String$(alignmentPadding - Len(formattedValue), PADDING_CHAR) & formattedValue                End If            End If            'Replace C# hex specifier with VB6 hex specifier,             'only if hex specifier was introduced in this function:            If (Not useLiteral And escapeHex) And _                StringContains(formattedValue, "0x") Then _            formattedValue = Replace$(formattedValue, "0x", "&H")            'replace all occurrences of placeholder {i} with their formatted values:            return_value = Replace(return_value, thisString, formattedValue, Count:=1)            'reset before reiterating:            thisFormat = vbNullString        NextcheckEscapes:        'if there's no more backslashes, don't bother checking for the rest:        If useLiteral Or Not StringContains(return_value, "\") Then GoTo normalExit        Dim escape As New EscapeSequence        Dim escapes As New Collection        escapes.Add escape.Create("\n", vbNewLine), "0"        escapes.Add escape.Create("\q", Chr$(34)), "1"        escapes.Add escape.Create("\t", vbTab), "2"        escapes.Add escape.Create("\a", Chr$(7)), "3"        escapes.Add escape.Create("\b", Chr$(8)), "4"        escapes.Add escape.Create("\v", Chr$(13)), "5"        escapes.Add escape.Create("\f", Chr$(14)), "6"        escapes.Add escape.Create("\r", Chr$(15)), "7"        For i = 0 To escapes.Count - 1            Set escape = escapes(CStr(i))            If StringContains(return_value, escape.EscapeString) Then _                return_value = Replace(return_value, escape.EscapeString, escape.ReplacementString)            If Not StringContains(return_value, "\") Then _                GoTo normalExit        Next        'replace "ASCII (oct)" escape sequence        Set regex = New RegExp        regex.Pattern = "\\(\d{3})"        regex.IgnoreCase = True        regex.Global = True        Set matches = regex.Execute(format_string)        Dim char As Long        If matches.Count <> 0 Then            For Each thisMatch In matches                p = thisMatch.SubMatches(0)                '"p" contains the octal number representing the ASCII code we're after:                p = "&O" & p 'prepend octal prefix                char = CLng(p)                return_value = Replace(return_value, thisMatch.Value, Chr$(char))            Next        End If        'if there's no more backslashes, don't bother checking for the rest:        If Not StringContains("\", return_value) Then GoTo normalExit        'replace "ASCII (hex)" escape sequence        Set regex = New RegExp        regex.Pattern = "\\x(\w{2})"        regex.IgnoreCase = True        regex.Global = True        Set matches = regex.Execute(format_string)        If matches.Count <> 0 Then            For Each thisMatch In matches                p = thisMatch.SubMatches(0)                '"p" contains the hex value representing the ASCII code we're after:                p = "&H" & p 'prepend hex prefix                char = CLng(p)                return_value = Replace(return_value, thisMatch.Value, Chr$(char))            Next        End IfnormalExit:        Set escapes = Nothing        Set escape = Nothing        If Not useLiteral And StringContains(return_value, Chr$(27)) Then _            return_value = Replace(return_value, Chr$(27), "\")        StringFormat = return_valueEnd Function

Notice the ParamArray in the method signature (thanks @wqw): doing so spares the usage of multiple optional parameters (and from usage bugs with being able to assign value2 without assigning value1 when naming the parameters in the calling statement). Because it's a ParamArray, the individual values are Variant which means every parameter could be of a different type, VB is doing the string conversion behind the scenes.

The function can then be consumed like this:

?StringFormat("(C) Currency: . . . . . . . . {0:C}\n" & _    "(D) Decimal:. . . . . . . . . {0:D}\n" & _    "(E) Scientific: . . . . . . . {1:E}\n" & _    "(F) Fixed point:. . . . . . . {1:F}\n" & _    "(N) Number: . . . . . . . . . {0:N}\n" & _    "(P) Percent:. . . . . . . . . {1:P}\n" & _    "(R) Round-trip: . . . . . . . {1:R}\n" & _    "(X) Hexadecimal:. . . . . . . {0:X}\n",-123, -123.45)

Output:

(C) Currency: . . . . . . . . -123.00$(D) Decimal:. . . . . . . . . -123(E) Scientific: . . . . . . . -1.23450E2(F) Fixed point:. . . . . . . -123(N) Number: . . . . . . . . . -123(P) Percent:. . . . . . . . . -12,345%(R) Round-trip: . . . . . . . -123.45(X) Hexadecimal:. . . . . . . &HFFFFFF85

And also like this:

?StringFormat("(c) Custom format: . . . . . .{0:cYYYY-MM-DD (MMMM)}\n" & _    "(d) Short date: . . . . . . . {0:d}\n" & _    "(D) Long date:. . . . . . . . {0:D}\n" & _    "(T) Long time:. . . . . . . . {0:T}\n" & _    "(f) Full date/short time: . . {0:f}\n" & _    "(F) Full date/long time:. . . {0:F}\n" & _    "(s) Sortable: . . . . . . . . {0:s}\n", Now())

Output:

(c) Custom format: . . . . . .2013-01-26 (January)(d) Short date: . . . . . . . 1/26/2013(D) Long date:. . . . . . . . Saturday, January 26, 2013(T) Long time:. . . . . . . . 8:28:11 PM(f) Full date/short time: . . 1/26/2013 8:28:11 PM(F) Full date/long time:. . . Saturday, January 26, 2013 8:28:11 PM(s) Sortable: . . . . . . . . 2013-01-26T20:28:11

Also possible to specify alignment (/padding) and to use escape sequences:

?StringFormat ("\q{0}, {1}!\x20\n'{2,10:C2}'\n'{2,-10:C2}'", "hello", "world", 100)"hello, world!"'   100.00$''100.00$   '

Looking at samples from http://msdn.microsoft.com/fr-fr/library/b1csw23d(v=vs.80).aspx, only a few format specifiers are not implemented, mostly date/time specifiers... but I would think the "c" custom date/time format specifier makes it up.

The function uses a straightforward implementation of String.Contains():

Public Function StringContains(string_source As String, find_text As String, _    Optional ByVal caseSensitive As Boolean = True) As Boolean    StringContains = StringContainsAny(string_source, caseSensitive, find_text)End Function

EDIT: This code now properly handles "\\" escapes, as mentioned in the comments. Also, while StringContains is certainly practical and gives a more comfortable reading than an InStr() call, the below StringContainsAny function is even better:

Public Function StringContainsAny(string_source As String, ByVal caseSensitive As Boolean, _    ParamArray find_values()) As Boolean    Dim i As Integer, found As Boolean    If caseSensitive Then        For i = LBound(find_values) To UBound(find_values)            found = (InStr(1, string_source, _                find_values(i), vbBinaryCompare) <> 0)            If found Then Exit For        Next    Else        For i = LBound(find_values) To UBound(find_values)            StringContainsAny = (InStr(1, LCase$(string_source), _                 LCase$(find_values(i)),   vbBinaryCompare) <> 0)            If found Then Exit For        Next    End If    StringContainsAny = foundEnd Function

Consider the following:

foo = Instr(1, source, "value1") > 0 Or Instr(1, source, "value2") > 0 _   Or Instr(1, source, "value3") > 0 Or Instr(1, source, "value4") > 0 _   Or Instr(1, source, "value5") > 0 Or Instr(1, source, "value6") > 0 _

Before VB can determine if foo is TRUE or FALSE, every single InStr() call is made. However with StringContainsAny(), the condition is satisfied with the first value that gets found, which makes it a faster statement.

EDIT: Previous edit pretty much wiped out escape sequences; reinstated them, using a small class "EscapeSequence" exposing two properties and a factory method - doing this allows keeping the for-each loop and handling all simple escapes without duplicating much code.

This code also uses a StringStartsWith function, implemented like this:

Public Function StringStartsWith(ByVal find_text As String, ByVal string_source As String, Optional ByVal caseSensitive As Boolean = True) As Boolean    If caseSensitive Then        StringStartsWith = (Left$(string_source, LenB(find_text) / 2) = find_text)    Else        StringStartsWith = (Left$(LCase(string_source), LenB(find_text) / 2) = LCase$(find_text))    End IfEnd Function