String Processing: StringCases and StringMatchQ

I recently got inconsistent results from StringCases and StringMatchQ (and used Pick instead of Cases). When using Cases or Select, you typically debug issues by mapping MatchQ onto the List that you're sifting for patterns. Similarly, StringMatchQ is the tool used to debug StringCases. However - while I don't believe there is any difference between patterns accepted by MatchQ vs. Cases, there is a significant difference between patterns accepted by StringMatchQ vs. StringCases.

StringMatchQ will accept what are called 'abbreviated String metacharacters'. These are the asterisk wildcard "*" for 0 or more characters, the @ wildcard for one or more non-upper case characters ("@@@@" matches any 4-non-upper case character Expression), and double-backslashes for literal characters (e.g. \\* to match an asterisk).

But StringCases will not; it will only accept StringExpression patterns or RegularExpressions. Here is an example. I want to extract all the "VCathode*" values from this datafile header:

data = {"%", "cln1x", "V", "(V)", "@", "VCathode=2.75392", "V", "(V)", "@", "VCathode=2.29025", "V", "(V)", "@", "VCathode=1.86092", "V", "(V)", "@", "VCathode=1.59846", "V", "(V)", "@", "VCathode=1.40122", "V", "(V)", "@", "VCathode=1.28137", "V", "(V)", "@", "VCathode=1.15876", "V", "(V)", "@", "VCathode=1.06451", "V", "(V)", "@", "VCathode=1.0037", "V", "(V)", "@", "VCathode=0.950678", "V", "(V)", "@", "VCathode=0.881652"};

StringCases with an asterisk fails:

In[190]:= StringCases[data,"VCathode*"]
Out[190]= {{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{},{}}

StringMatchQ with an asterisk works:

In[191]:= StringMatchQ[data,"VCathode*"]
Out[191]= {False,False,False,False,False,True,False,False,False,True,False,False,False,True,False,False,False,True,False,False,False,True,False,False,False,True,False,False,False,True,False,False,False,True,False,False,False,True,False,False,False,True,False,False,False,True}

Here is the solution for StringCases using StringExpressions. I use Flatten to remove nulls ("{}").

In[192]:= StringCases[#,"VCathode"~~___]&/@data//Flatten
Out[192]= {VCathode=2.75392,VCathode=2.29025,VCathode=1.86092,VCathode=1.59846,VCathode=1.40122,VCathode=1.28137,VCathode=1.15876,VCathode=1.06451,VCathode=1.0037,VCathode=0.950678,VCathode=0.881652}

And here is the solution for StringCases using a RegularExpression. Here I use DeleteCases instead of Flatten to remove nulls, but leave sets of List brackets around the returned values.

In[195]:= StringCases[#,RegularExpression["VCathode.*"]]&/@data//DeleteCases[#,{}]&
Out[195]= {{VCathode=2.75392},{VCathode=2.29025},{VCathode=1.86092},{VCathode=1.59846},{VCathode=1.40122},{VCathode=1.28137},{VCathode=1.15876},{VCathode=1.06451},{VCathode=1.0037},{VCathode=0.950678},{VCathode=0.881652}}

Thank you to Xin Xiao from Wolfram Technical Support.

String Replacement Methods: StringForm

A String Replacement Overview is here.

StringForm

StringForm is a simple, elegant String template function. Use it in your functions where Print isn't enough since you need to fill in some variables, such as calculations on the fly. In a function use the following form with Print to output it since lines end with an output-suppressing semi-colon.

Print@StringForm["control string", variables];

Here the double backtick marks tell Mathematica where to fill in the blanks with arguments you give in the order in which they are inserted into the String. An argument can be a String or an Expression of unlimited complexity, which will be evaluated before insertion. If you don't want the inserted Expression to be evaluated, though, use HoldForm (example below).

StringForm["Use `` for relatively short and simple String templates, such as output messages in your functions. For example, the cube root of `` is ``.","StringForm",27,27^(1/3)]

Use StringForm for relatively short and simple String templates, such as output messages in your functions. For example, the cube root of 27 is 3.

If you're going to use an argument twice, switch the order, or use a number of arguments and you want to prevent mistakes, use numbered, rather than ordered, backticks. You often want a line break, for which the \n escape character is used within the quotation marks that are Mathematica's String delimiters.

StringForm["Flying or gliding mammals include `1`, `2`,\n`3`, `4`, `5`, `6`, and `7`.\nThe most common species are in the `3` family.","flying possums","greater glider","bats","flying squirrels","flying lemurs","flying monkeys","cats"]

Flying or gliding mammals include flying possums, greater glider,
bats, flying squirrels, flying lemurs, flying monkeys, and cats.
The most common species are in the bats family.

To prevent the inserted Expression from being evaluated, use HoldForm:

StringForm["For example, the sixth term of the Fibonacci series is the sum of the preceding two terms: ``.",HoldForm[1+1+2+3+5=8]]


For example, the sixth term of the Fibonacci series is the sum of the preceding two terms: 1+1+2+3+5=8.

Create Predicates to Test Strings vs Numerics: Test for File Extensions

Elsewhere I tell why predicates are important to use, show how to find all predicates in Mathematica including ones not suffixed with "Q", and create ones of your own to test numerical expressions. Here I cover predicates for testing Strings, and create some tests for file types, as judged by their extensions. Note that FileExtension does not capture the period before the extension.

Equal (==) and SameQ (===) are valid predicates for Strings as well as numerics, and work fine for simple applications.

In[206]:= uncusFileQ@fileName_String := FileExtension@fileName == "unc"

In[207]:= uncusFileQ@"afile.unc"

Out[207]= True

In[208]:= uncusFileQ@"afile.txt"

Out[208]= False

In[209]:= textFileQ@fileName_String := FileExtension@fileName == "txt"

In[215]:= textFileQ@"sampleFile.txt"

Out[215]= True

To test for two alternative file extensions is a little trickier. Since we are testing for String equivalence between String patterns, we use the general String predicate function StringMatchQ and Alternatives (|), not logical Or (||). An example is testing for HTML files, since their extensions come in .htm and .html flavors.

In[210]:= Clear@htmlFileQ;
htmlFileQ@fileName_String := StringMatchQ[FileExtension@fileName, "html" | "htm"]

In[212]:= htmlFileQ@"www.jognog.com/index.html"

Out[212]= True

In[213]:= htmlFileQ@"www.jognog.com/index.htm"

Out[213]= True

In[214]:= htmlFileQ@"www.jognog.com/sitemap.xml"

Out[214]= False