Give a regular expression for all strings whose length is strictly greater than 3.

A regular expression that matches all strings whose length is strictly greater than 3 can be written as follows:

^.{4,}$

Here's the step-by-step explanation:

1. ^ : This symbol is used to indicate the start of a line.

2. . : This is a special character in regular expressions that matches any character except a newline.

3. {4,} : This is a quantifier in regular expressions. The number 4 indicates the minimum number of times the preceding element (in this case, any character) should match. The comma following the 4 indicates that there is no upper limit on the number of matches.

4. $ : This symbol is used to indicate the end of a line.

So, ^.{4,}$ will match any string that has at least 4 characters.