BASICS
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^	Match the beginning of the string
$	Match the end of the string 
.	Match any character except newline
\w	Match any alphanumeric character
\s	Match any whitespace character
\d	Match any digit
\b	Match the beginning or end of a word

REPITION
--------
*		Repeat any number of times
+		Repeat one or more times
?		Repeat zero or one time
{n}		Repeat n times
{n,m}	Repeat at least n, but no more than m times
{n,}	Repeat at least n times

NEGATION
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\W		Match any character that is NOT alphanumeric
\S		Match any character that is NOT whitespace
\D		Match any character that is NOT a digit
\B		Match a position that is NOT the beginning or end of a word
[^x]	Match any character that is NOT x
[^abc]	Match any character that is NOT one of the characters abc 

ALTERNATIVES
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|	Pipe symbol separates alternatives. Evaluation is from left to right

GROUPING
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()	A Group should be enclosed within brackets

CAPTURES
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(exp)			Match exp and capture it in an automatically numbered group
(?<name>exp)	Match exp and capture it in a group named name
(?:exp)			Match exp, but do not capture it 


LOOKAROUNDS
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(?=exp)		Match any position preceding a suffix exp
(?<=exp)	Match any position following a prefix exp
(?!exp)		Match any position after which the suffix exp is not found
(?<!exp)	Match any position before which the prefix exp is not found


COMMENT
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(?#comment)		Comment 


POSITIVE LOOKAROUND
-------------------
The next four are so-called lookahead or lookbehind assertions. They look for things that go before or after the current match without including them in the match. It is important to understand that these expressions match a position like "^" or "\b" and never match any text. For this reason, they are known as "zero-width assertions". They are best illustrated by example:

"(?=exp)" is the "zero-width positive lookahead assertion". It matches a position in the text that precedes a given suffix, but doesn't include the suffix in the match:

Eg1. \b\w+(?=ing\b) The beginning of words ending with "ing"

"(?<=exp)" is the "zero-width positive lookbehind assertion". It matches the position following a prefix, but doesn't include the prefix in the match:

Eg2. (?<=\bre)\w+\b The end of words starting with "re"

Here is an example that could be used repeatedly to insert commas into numbers in groups of three digits:

Eg3. (?<=\d)\d{3}\b Three digits at the end of a word, preceded by a digit

Here is an example that looks for both a prefix and a suffix:

Eg4. (?<=\s)\w+(?=\s) Alphanumeric strings bounded by whitespace

NEGATIVE LOOKAROUND
-------------------
Earlier, I showed how to search for a character that is not a specific character or the member of a character class. What if we simply want to verify that a character is not present, but don't want to match anything? For example, what if we are searching for words in which the letter "q" is not followed by the letter "u"? We could try:

Eg1. \b\w*q[^u]\w*\b Words with "q" followed by NOT "u"

Run the example and you will see that it fails when "q" is the last letter of a word, as in "Iraq". This is because "[^q]" always matches a character. If "q" is the last character of the word, it will match the whitespace character that follows, so in the example the expression ends up matching two whole words. Negative lookaround solves this problem because it matches a position and does not consume any text. As with positive lookaround, it can also be used to match the position of an arbitrarily complex subexpression, rather than just a single character. We can now do a better job:

Eg2. \b\w*q(?!u)\w*\b Search for words with "q" not followed by "u"

We used the "zero-width negative lookahead assertion", "(?!exp)", which succeeds only if the suffix "exp" is not present. Here is another example:

Eg3. \d{3}(?!\d) Three digits not followed by another digit

Similarly, we can use "(?<!exp)", the "zero-width negative lookbehind assertion", to search for a position in the text at which the prefix "exp" is not present:

Eg4. (?<![a-z ])\w{7} Strings of 7 alphanumerics not preceded by a letter or space

Here is one more example using lookaround:

Eg5. (?<=<(\w+)>).*(?=<\/\1>) Text between HTML tags

This searches for an HTML tag using lookbehind and the corresponding closing tag using lookahead, thus capturing the intervening text but excluding both tags.

GREEDY AND LAZY
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Repition quantifier default to "greedy" matching. Follow a quantifier with a ? mark to enforce "lazy" matching.

*?		Repeat any number of times, but as few as possible
+?		Repeat one or more times, but as few as possible
??		Repeat zero or one time, but as few as possible
{n,m}?	Repeat at least n, but no more than m times, but as few as possible
{n,}?	Repeat at least n times, but as few as possible