Strings are significant to the grammar so only use single quotes where they are needed. 2 Turn off all interpretation of special characters and keywords, just like double quotes. Braces have syntatical significance, delimiting lists. Wherever you can use braces you can use angle brackets. Parentheses don't turn off any sort of expansion, but can have syntactical significance.
Expansion is turned off within single and double quotes and within braces.
Because the expansion happens before the parser sees the input, variables cannot have any syntactical significance as the parser never knows about them. Evaluate the expression expr and substitute the resulting value.