Re: Pattern matching question and answer
- To: mathgroup at smc.vnet.net
- Subject: [mg63062] Re: Pattern matching question and answer
- From: dh <dh at metrohm.ch>
- Date: Tue, 13 Dec 2005 03:40:43 -0500 (EST)
- References: <dnir79$9ur$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Hi Matt, your problem is that you want to use the machanism of pattern matching but you specify your condition not as a pattern, but as a string-pattern. One way to solve this problem would be to use pattern matching, but attaching a condition to the pattern. This condition can then contain the string-pattern. E.g. Cases[expr, _ x_ /; StringMatchQ[ ToString[x], CharacterRange["x", "z"] ~~ "0"]] Daniel Matt wrote: > 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 >