This code works fine in WolframScript: x_.. is interpreted as a Repeated form of the pattern x_,

StringCases["hello", x_..] (* {"h", "e", "ll", "o"} *) 

Try it online!

In the front end, however, it fails because x_. is interpreted as a Default pattern and the second dot throws a syntax error. The code must be modified to

StringCases["hello", (x_)..] (* {"h", "e", "ll", "o"} *) 

Why is there a difference in behavior between the front end and direct kernel access (WolframScript)?

This code works fine in a string based interface like WolframScript or Mathematica Kernel.

Run this through a string based interface

StringCases["hello", x_..] 

Or do

ToExpression["StringCases[\"hello\", x_..]"] 

in the notebook. Both work correctly, x_.. is interpreted as a Repeated form of the pattern x_,

Try it online!

In the notebook based front end, however, it fails because x_. is interpreted as a Default pattern and the second dot throws a syntax error.

The code must be modified to

StringCases["hello", (x_)..] (* {"h", "e", "ll", "o"} *) 

Why is there a difference in behavior between the front end and kernel parsers.

This code works fine in WolframScript V11.3.0: x_.. is interpreted as a Repeated form of the pattern x_,

StringCases["hello", s:x_..] (* {"h", "e", "ll", "o"} *) 

Try it online!

In Mathematica 12.0.0, however, it fails because x_. is interpreted as a Default pattern and the second dot throws a syntax error. The code must be modified to

StringCases["hello", s:(x_)..] (* {"h", "e", "ll", "o"} *) 

What exactly has changed? I cannot find a difference in the Precedence values between the versions. How can the lexer be spelunked?

This code works fine in WolframScript: x_.. is interpreted as a Repeated form of the pattern x_,

StringCases["hello", x_..] (* {"h", "e", "ll", "o"} *) 

Try it online!

In the front end, however, it fails because x_. is interpreted as a Default pattern and the second dot throws a syntax error. The code must be modified to

StringCases["hello", (x_)..] (* {"h", "e", "ll", "o"} *) 

Why is there a difference in behavior between the front end and direct kernel access (WolframScript)?