inputdlg - how do i recognise if someone entered a symbol, e.g. = into the box?

how do i recognise if someone entered a symbol, e.g. = into the box?

[Stephen23]

What counts as a "symbol"? Is e a symbol? Is - a symbol? What about -1e3 ? What about i ? Or 1+i ?

You need to specify what a symbol is, or is not.

[Dominique Joubert]

!@#$%&=

[Dominique Joubert]

i want a user to enter the right side of an equation:for example s+2*a-3/r

[Stephen23]

What about all of the ASCII control characters, i.e. char(0:31)? What about non-breaking spaces, en dashes, em dashes, etc?

There are many characters that you are likely to want to exclude, and building a list of them all is intractable. It would be much easier to define what characters are allowed, as this list will be much smaller. For that you can use a regular expression.

Note that writing a language parser is not a trivial task.

Answer 1

[Niklas Nylén]

Verifying that the expression is a valid equation is probably extremely hard, but validating that only "allowed" symbols are entered is quite simple using regexp:

% Allowing +, -, *, / and space. Expand list if needed.
validSpecialChars = regexptranslate('escape', '+-*/ ')

assert(isempty(regexp(input, ['[^A-Za-z0-9' validSpecialChars ']'], ...
  'Expression %s contains invalid characters', input)