What is the problem with the "|" character inside of strings?

Question

When the "|" character appears inside of a string containing linear box syntax, SyntaxQ returns False and FrontEnd often fails to display the string correctly. Consider:

1. A string with an alphabetical character embedded in linear box syntax, it's OK:

   "\!\(\*StyleBox[\"a\"]\)" // SyntaxQ
   

   True
   

2. When we put the "|" character (which is ASCII!), something goes wrong:

   "\!\(\*StyleBox[\"|\"]\)" // SyntaxQ
   

   False
   

3. Well, let us take what Mathematica suggests:

   str = ToString[Style["|"], StandardForm];
   str // SyntaxQ
   str // InputForm
   

   True
   
   "\!\(\*StyleBox[\"\\\"|\\\"\", Rule[StripOnInput, False]]\)"
   

Now it looks OK, but I wish to have additional "|" in the string, so I try:

str <> "|" // SyntaxQ

False

Again something goes wrong (but why???), while with the alphabetical characters it's OK:

str <> "a" // SyntaxQ

True

StringJoin of str with itself is also OK, but I don't wish to blow up my file having every "|" character wrapped by the linear syntax. The above is just a minimal working example, of course. Actually I'm constructing a string containing both Bold and Plain (the default style) symbols "|".

I'm working with large strings and wish to keep them as small in size as possible and I'm also trying to make the script as efficient as possible, hence I wish to avoid excessive linear syntax inside of the strings where it isn't strictly necessary.

What is the correct way to compose a large string with differently styled "|" characters? The string have to be as small as possible, with default formatting applied to unstyled characters.

Answer 1

The basic issue is that | corresponds to an operator, and not a letter. A simpler example:

SyntaxQ["a"]
SyntaxQ["|"]

True

False

SyntaxQ means that the string can be parsed into a Mathematica expression (e.g., ToExpression). You can't expect a string to be syntactically correct when you add an operator in a random place. Can you just use "\"|\"" instead of "|"?

In order to have SyntaxQ strings that can be string joined, you probably need to create linear syntax for each string. So, something like:

plain = "\!\(\"|\"\)"
bold = "\!\(\*StyleBox[\"\\\"|\\\"\", Bold]\)"
bold <> plain

plain //SyntaxQ
bold //SyntaxQ
bold <> plain //SyntaxQ

Here's a pic: