Error checking using regex ?

Guy Robinson guy at NOSPAM.r-e-d.co.nz
Tue Jun 8 23:10:51 CEST 2004


Hi Paul,

Yep your examples :-) I'm using this as a learning experience and have 
looked at your code but I have specific requirements for integration 
into another application.

I'm using the regex to create a list of tokens to be processed into a 
postfix processing string. This is then offloaded to another class that 
processes the string for each database row.

The speed to generate the postffix string isn't important. But the speed 
to process for each database row is.

Guy

> "Guy Robinson" <guy at NOSPAM.r-e-d.co.nz> wrote in message
> news:ca47pc$e11$1 at lust.ihug.co.nz...
> 
>>I have the code below which parses an expression string and creates
> 
> tokens.
> 
>>Can anyone suggest the best of error checking for things like:
>>
>>Valid variable only obj.attribute -whitespace allowed
>>
>>test( "ff*2/dd.r..ss r")    #additional ..ss -invalid variable.
>>test( "ff*$24..55/ddr")     #double .. and $ -invalid number
>>test( "ff*2/dd.r.ss r")     #variable with double . -invalid variable
>>
>>I can't see an efficient way of doing this so any suggestions appreciated.
>>
>>TIA,
>>
>>Guy
>>
> 
> <snip>
> 
> Guy -
> 
> Well, I recognize the test cases from an example that I include with
> pyparsing.  Are you trying to add support for variables to that example?  If
> so, here is the example, modified to support assignments to variables.
> 
> -- Paul
> 
> ============================
> # minimath.py (formerly fourfn.py)
> #
> # Demonstration of the parsing module, implementing a simple 4-function
> expression parser,
> # with support for scientific notation, and symbols for e and pi.
> # Extended to add exponentiation and simple built-in functions.
> # Extended to add variable assignment, storage, and evaluation, and
> Python-like comments.
> #
> # Copyright 2003,2004 by Paul McGuire
> #
> from pyparsing import
> Literal,CaselessLiteral,Word,Combine,Group,Optional,ZeroOrMore,OneOrMore,For
> ward,nums,alphas,restOfLine,delimitedList
> import math
> 
> variables = {}
> exprStack = []
> 
> def pushFirst( str, loc, toks ):
>     global exprStack
>     if toks:
>         exprStack.append( toks[0] )
>     return toks
> 
> def assignVar( str, loc, toks ):
>     global exprStack
>     global variables
>     variables[ toks[0] ] = evaluateStack( exprStack )
>     pushFirst(str,loc,toks)
> 
> 
> bnf = None
> def BNF():
>     global bnf
>     if not bnf:
>         point = Literal( "." )
>         e     = CaselessLiteral( "E" )
>         fnumber = Combine( Word( "+-"+nums, nums ) +
>                            Optional( point + Optional( Word( nums ) ) ) +
>                            Optional( e + Word( "+-"+nums, nums ) ) )
>         ident = Word(alphas, alphas+nums+"_$")
>         varident = delimitedList(ident,".",combine=True)
> 
>         plus  = Literal( "+" )
>         minus = Literal( "-" )
>         mult  = Literal( "*" )
>         div   = Literal( "/" )
>         lpar  = Literal( "(" ).suppress()
>         rpar  = Literal( ")" ).suppress()
>         addop  = plus | minus
>         multop = mult | div
>         expop = Literal( "^" )
>         pi    = CaselessLiteral( "PI" )
> 
>         expr = Forward()
>         atom = ( pi | e | fnumber | ident + lpar + expr + rpar |
> varident ).setParseAction( pushFirst ) | ( lpar + expr.suppress() + rpar )
>         factor = atom + ZeroOrMore( ( expop + expr ).setParseAction(
> pushFirst ) )
>         term = factor + ZeroOrMore( ( multop + factor ).setParseAction(
> pushFirst ) )
>         expr << term + ZeroOrMore( ( addop + term ).setParseAction(
> pushFirst ) )
>         assignment = (varident + "=" + expr).setParseAction( assignVar )
> 
>         bnf = Optional( assignment | expr )
> 
>         comment = "#" + restOfLine
>         bnf.ignore(comment)
> 
>     return bnf
> 
> # map operator symbols to corresponding arithmetic operations
> opn = { "+" : ( lambda a,b: a + b ),
>         "-" : ( lambda a,b: a - b ),
>         "*" : ( lambda a,b: a * b ),
>         "/" : ( lambda a,b: a / b ),
>         "^" : ( lambda a,b: a ** b ) }
> fn  = { "sin" : math.sin,
>         "cos" : math.cos,
>         "tan" : math.tan,
>         "abs" : abs,
>         "trunc" : ( lambda a: int(a) ),
>         "round" : ( lambda a: int(a+0.5) ),
>         "sgn" : ( lambda a: ( (a<0 and -1) or (a>0 and 1) or 0 ) ) }
> def evaluateStack( s ):
>     global variables
>     if not s: return 0.0
>     op = s.pop()
>     if op in "+-*/^":
>         op2 = evaluateStack( s )
>         op1 = evaluateStack( s )
>         return opn[op]( op1, op2 )
>     elif op == "PI":
>         return 3.1415926535
>     elif op == "E":
>         return 2.718281828
>     elif op[0].isalpha():
>         if op in variables:
>             return variables[op]
>         fnarg = evaluateStack( s )
>         return (fn[op])( fnarg )
>     else:
>         return float( op )
> 
> if __name__ == "__main__":
> 
>     def test( str ):
>         global exprStack
>         exprStack = []
>         results = BNF().parseString( str )
>         print str, "->", results, "=>", exprStack, "=", evaluateStack(
> exprStack )
> 
>     test( "9" )
>     test( "9 + 3 + 6" )
>     test( "9 + 3 / 11" )
>     test( "(9 + 3)" )
>     test( "(9+3) / 11" )
>     test( "9 - 12 - 6" )
>     test( "9 - (12 - 6)" )
>     test( "2*3.14159" )
>     test( "3.1415926535*3.1415926535 / 10" )
>     test( "PI * PI / 10" )
>     test( "PI*PI/10" )
>     test( "PI^2" )
>     test( "6.02E23 * 8.048" )
>     test( "e / 3" )
>     test( "sin(PI/2)" )
>     test( "trunc(E)" )
>     test( "E^PI" )
>     test( "2^3^2" )
>     test( "2^9" )
>     test( "sgn(-2)" )
>     test( "sgn(0)" )
>     test( "sgn(0.1)" )
>     test( "5*4+300/(5-2)*(6+4)+4" )
>     test( "((5*4+301)/(5-2))*(6+4)+4" )
>     test( "(321/3)*10+4" )
>     test( "# nothing but comments" )
>     test( "a = 2^10" )
>     test( "a^0.1   # same as 10th root of 1024" )
>     test( "c = a" )
>     test( "b=a" )
>     test( "b-c" )
> 
> 



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