Include the validation of templates in the SHACL shapes

[chrdebru]

I believe that checking the validity of templates should be included in the shapes. I'm not sure whether SPARQL's regular expressions allow for recursion, but it can be achieved by:

• removing the unescaped curly braces from the template
• checking whether the resulting string matches ^[^\{\}]*(?:\{[^\{\}]+\}[^\{\}]*)*$ (balanced and not nested)

This can be achieved for a SPARQL constraint component.

[DylanVanAssche]

SHACL can do regex, once the listed regex is tested and test cases are provided with valid and invalid templates, we can add it.

[dachafra]

Tasks required:

• Test the regex provided and ensure that the constraint is correct
• Define test-cases with valid templates (I think this is by default now and it's not required)
• Define a few test-cases with invalid templates.

@DylanVanAssche could you take care of this?

[DylanVanAssche]

Define a few test-cases with invalid templates.
AFAIK, these are the 'rules':

A string template is a format string that can be used to build strings from multiple components. It can apply [reference expressions](https://kg-construct.github.io/rml-core/spec/docs/#dfn-reference-expression) by enclosing them in curly braces ({ and }). The following syntax rules apply to valid [string templates](https://kg-construct.github.io/rml-core/spec/docs/#dfn-string-template):

    1. Pairs of unescaped curly braces MUST enclose valid [reference expressions](https://kg-construct.github.io/rml-core/spec/docs/#dfn-reference-expression).
   2. Curly braces that do not enclose [reference expressions](https://kg-construct.github.io/rml-core/spec/docs/#dfn-reference-expression) MUST be escaped by a backslash character (\). This also applies to curly braces within [reference expressions](https://kg-construct.github.io/rml-core/spec/docs/#dfn-reference-expression).
    3.Backslash characters (\) MUST be escaped by preceding them with another backslash character, yielding (\\). This also applies to backslashes within [reference expressions](https://kg-construct.github.io/rml-core/spec/docs/#dfn-reference-expression).
    4.There SHOULD be at least one pair of unescaped curly braces.

from spec.

Anything missing?
If not, we need to have 4 invalid test cases, one for each rule.

[dachafra]

I don't think so, please proceed :-)