What is a character class? How to specify a character class in a regular expression?

How to write a regular expression using character classes to capture 7 digits phone numbers from this example string "123-4567, 123 4567 or 1234567"?

A character class allows a set of possible characters, rather than just a single character, to match at a particular point in a regex. Character classes are denoted by brackets [...] , with the set of characters to be possibly matched inside.

Here are some examples of character classes:

[bcr]at        # matches 'bat', 'cat', or 'rat'
[yY][eE][sS]   # match 'yes' in a case-insensitive way

The regular expression using character classes to capture 7-digit phone numbers from a text file:

[0123456789]{3}[- ]?[0123456789]{4}

[0123456789] - character class of any decimal digits
{3} - repeating 3 times
[- ]? - '-', ' ' or no character