Re: When a string does not match itself

*To*: mathgroup at smc.vnet.net*Subject*: [mg97231] Re: When a string does not match itself*From*: "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>*Date*: Mon, 9 Mar 2009 01:01:52 -0500 (EST)*References*: <got8h2$go0$1@smc.vnet.net>

Hi Ingolf, My explanation would be the following The first "\\*" is a standard string. In such a string the '\' is an escape character. For instance, "\n" is the newline character (or perhaps the lf+cr, I forgot). It's a single character, with a single character code. To be able to input the '\' in a string as a literal character it can be escaped by itself. So, "\\" is the character '\'. Therefore, "\\*" is the concatenation of the characters '\' and '*'. The second argument of StringMatchQ can be a substring or a string pattern. The latter may be present in an abbreviated form. In this form, "*" stands for zero or more characters. If you want to match with a literal '*', you'd have a problem as "\*" would be the escaped character '*', which is a single character with character code 63432 (as can be checked with ToCharacterCode["\*"]). So, in an abbreviated pattern string '*' has to be escaped twice, before the pattern is interpreted as '*', i.e. as "\\*". This is documented in the "More Information" section of StringMatchQ's documentation. Therefore, StringMatchQ["\\*","\\*"] compares the literal characters '\' + '*' with the literal '*', which of course returns False. StringCases and StringPosition don't have the abbreviated string pattern option. The can only use substrings, StringExpression and RegularExpression. That's why they interpret the second "\ \*"differently. To have StringMatchQ match the two strings you could use Verbatim: In[112]:= StringMatchQ["\\*", Verbatim["\\*"]] Out[112]= True Cheers -- Sjoerd On Mar 7, 9:42 am, <ingolf.d... at telia.com> wrote: > This is a maybe a trivial question, but I have not found the explanation = in > help (I am using Mathematica 7.0.0): > > Normally a string matches itself: > > In[270]:= StringMatchQ["monkey", "monkey"] > > Out[270]= True > > But > > In[271]:= StringMatchQ["\\*", "\\*"] > > Out[271]= False > > Why? Compare also to > > In[4]:= StringCases["\\*", "\\*"] > > Out[4]= {"\\*"} > > In[267]:= StringPosition["\\*", "\\*"] > > Out[267]= {{1, 2}} > > Best regards > > Ingolf Dahl > > Sweden > > ingolf.d... at telia.com