\ is used to escape the following character when that character is a special character. So, for example, a regular expression that found .com would be \.com because . is a special character that matches any character.
\d matches any single digit
\w matches any part of word character (equivalent to [A-Za-z0-9])
\s matches any space, tab, or newline
^ asserts the position at the start of the line. So what you put after it will only match if they are the first characters of a line.
$ asserts the position at the end of the line. So what you put before it will only match if they are the last characters of a line.
\b adds a word boundary. Putting this either side of a stops the regular expression matching longer variants of words.
* matches the preceding element zero or more times. For example, ab*c matches “ac”, “abc”, “abbbc”, etc.
+ matches the preceding element one or more times. For example, ab+c matches “abc”, “abbbc” but not “ac”.
? matches when the preceding character appears one or zero times
{VALUE} matches the preceding character the number of times define by VALUE; ranges can be specified with the syntax {VALUE,VALUE}