How can I check if a single character appears in a string?

You can use system expressions and check TextInput.Text for occurences.

Let TextInput.Text be "cool guy :D"

Option 1)

find(Text,Find) will find you a string inside another string.

So find(TextInput.Text,";") should return -1.

Option 2)

The system expression tokencount will return the number of tokens in a string

tokencount(TextInput.Text,";") should return 0.

Option 3)

The system expression tokenat returns the Nth token from a string, hence -1 if no occurence.

tokenat(TextInput.Text,";") should therefore return -1.

If one of the methods matches its condition, clear Textinput