.

The dot or period character represents any character except new line character.

do. matches doe, dog, don, dos, dot, etc.d.r matches deer, door, etc.

*

The asterisk character means zero or more instances of the preceding element.

do* matches d, do, doo, dooo, doooo, etc.

+

The plus sign character means one or more instances of the preceding element.

do+ matches do, doo, dooo, doooo, etc. but not d

?

The question mark character means zero or one instances of the preceding element.

do?g matches dg or dog but not doog, dooog, etc.

( )

Parenthesis characters group whatever is between them to be considered as a single entity.

d(eer)+ matches deer or deereer or deereereer, etc. The + sign is applied to the substring within parentheses, so the regex looks for d followed by one or more of the grouping "eer." 

[ ]

Square bracket characters indicate a set or a range of characters.

d[aeiouy]+ matches da, de, di, do, du, dy, daa, dae, dai, etc. The + sign is applied to the set within brackets parentheses, so the regex looks for d followed by one or more of any of the characters in the set [aeioy].

d[A-Z] matches dA, dB, dC, and so on up to dZ. The set in square brackets represents the range of all upper-case letters between A and Z.

^

Carat characters within square brackets logically negate the set or range specified, meaning the regex will match any character that is not in the set or range.

d[^aeiouy] matches db, dc or dd, d9, d#. d followed by any single character except a vowel.

{ }

Curly brace characters set a specific number of occurrences of the preceding element. A single value inside the braces means that only that many occurrences will match. A pair of numbers separated by a comma represents a set of valid counts of the preceding character. A single digit followed by a comma means there is no upper bound.

da{3} matches daaa. d followed by 3 and only 3 occurrences of ”r;a”. da{2,4} matches daa, daaa, daaaa, and daaaa (but not daaaaa). d followed by 2, 3, or 4 occurrences of ”r;a”. da{4,} matches daaaa, daaaaa, daaaaaa, etc. d followed by 4 or more occurrences of ”r;a”.