Re: Pattern matching question and answer

*To*: mathgroup at smc.vnet.net*Subject*: [mg63069] Re: [mg63054] Pattern matching question and answer*From*: Bob Hanlon <hanlonr at cox.net>*Date*: Tue, 13 Dec 2005 03:40:51 -0500 (EST)*Reply-to*: hanlonr at cox.net*Sender*: owner-wri-mathgroup at wolfram.com

expr=y0*y^2+z0*z^2+x0*x^2+a1*a^2+b1*b^2+q0; var=ToExpression[#<>"0"&/@ Cases[CharacterRange["A","z"],_?LetterQ]]; Flatten[Cases[expr,#*_.,1]&/@var] {q0, x^2*x0, y^2*y0, z^2*z0} Note use of _. to handle default (e.g., q0 term). Bob Hanlon > > From: "Matt" <anonmous69 at netscape.net> To: mathgroup at smc.vnet.net > Date: 2005/12/11 Sun PM 10:25:31 EST > Subject: [mg63069] [mg63054] Pattern matching question and answer > > Hello, > I recently was going to post this question: > > *******QUESTION START******* > > Is there a way to specify a pattern within an expression that can > specify certain alpha-numeric combinations? > > e.g. > > expr = y0*y^2 + z0*z^2 + x0*x^2 + a1*a^2 + b1*b^2; > > If I wanted to use Cases (or whatever is deemed appropriate to > accomplish the task) to return a list of only the parts of expr that > are multiplied by a variable of the form 'letter'zero (i.e. x0,y0,z0), > how would I do this? > > I could do this: > Cases[expr, Times[x0,_]] > Cases[expr, Times[y0,_]] > Cases[expr, Times[z0,_]] > > But what I'm looking for would be a pattern that specifies a single > character followed by a zero. In regular expression syntax, if I were > dealing with a text file, I might use something like this: > [A-Za-z]0 > > or like this > > [xyz]0 > > So, my Cases statement would be able to grab all three matches like > this (if it were supported): > Cases[expr, Times[[xyz]0,_]] > > *******QUESTION END******* > > But before I got around to posting, I came up with a solution. First > off, once again, I'm impressed with Mathematica's flexibility, but I was > wondering if there's better or more intuitive way to accomplish the > task? > > Solution: > Remove[expr, x0, x, y0, y, z0, z] > expr = x0*x^2 + x00*x0^2 + y0*y^2 + z0*z^2 + x0*Sin[x] > Cases[expr, _^_*(x:_Symbol?(StringMatchQ[ToString[#1], > RegularExpression["[xy]0"]] & ))] > > I could also use this: > Cases[expr, _*(x:_Symbol?(StringLength[ToString[#1]] == 2 & ))] > > if I were interested in identifiers of length 2. > > Thanks, > > Matt >