[regex] What is the regular expression to allow uppercase/lowercase (alphabetical characters), periods, spaces and dashes only?

The regex you're looking for is ^[A-Za-z.\s_-]+$

• ^ asserts that the regular expression must match at the beginning of the subject
• [] is a character class - any character that matches inside this expression is allowed
• A-Z allows a range of uppercase characters
• a-z allows a range of lowercase characters
• . matches a period rather than a range of characters
• \s matches whitespace (spaces and tabs)
• _ matches an underscore
• - matches a dash (hyphen); we have it as the last character in the character class so it doesn't get interpreted as being part of a character range. We could also escape it (\-) instead and put it anywhere in the character class, but that's less clear
• + asserts that the preceding expression (in our case, the character class) must match one or more times
• $ Finally, this asserts that we're now at the end of the subject

When you're testing regular expressions, you'll likely find a tool like regexpal helpful. This allows you to see your regular expression match (or fail to match) your sample data in real time as you write it.