In numbers and more i don't know what a "valid expression" is, who can please help me??
A valid expression is any numeric or string expression that follows the compiler expression rules, or logic expresson for that matter.
This applies to any programming language, not just to JS.
Look it up on the WWW. Do not let this site be your only means of exploring the wonderful world of programming.
Here is an example of a mostly universal valid expression in C, Java, Python, Pascal and probably a lot more
BTW, I just realised, you arn't mistakenly calling Regular Expressions as Valid Expressions are you ?
a = c + d;
Where a, c and d are previously declared as integers.
This is also an algebraic expression. The letters represent Integer values.
You can have a string expression
myName = "Donald"