Question

replace(/[^0-9]/g,''));

1. Replace is a method
2. What does / indicate?
3. What does ^ indicate along with 0-9
4. What does /g indicate?

Do we need to start a regular expression with / or can we start with anything?

Answer 1

The / introduces a regular expression literal (just like " and ' introduce string literals). A regular expression literal is in the form /expression/flags, where expression is the body of the expression, and flags are optional flags (i for case-insensitive, g for global, m for multi-line stuff).

The ^ as the first character within [] means any character not matching the following. So [^0-9] means “any character except 0 through 9“.

The /g ends the regular expression literal and includes the “global” flag on it. Without the g, replace would only replace the first match, not all of them.

In all, what that does is replace any character that isn’t 0 through 9 with a blank — e.g., removes non-digits. It could be written more simply as:

var result = str.replace(/\D/g, '');

…because \D (note that’s an upper-case D) means “non-digit”.

MDC has a decent page on regular expressions.