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# Capture an entire line that contains a word
(?=.*WORD).*

# Capture an entire line that contains two words in any order
(?=.*WORD1)(?=.*WORD2).*

# Capture entire lines that contain either WORD[1-3] from left to right
(?=.*WORD1|WORD2|WORD3).*

Capture entire lines that contain WORD[1-3] from left to right 
(.*?=WORD1|WORD2|WORD3).*

# Capture every word in the string EXCEPT the words in the list
(?>[\w-]+)(?<!WORD1|WORD2|WORD3|WORD4)

# Capture every part of the string that comes before WORD
.*(?=WORD)

# Capture every part of the string that comes after WORD
(?<=WORD).*

# Capture every part of the string that comes between two words
(?<=WORD1).*(?=WORD2)

# Captures the first word after WORD
(?<=WORD)\s+(\w+)

# Captures the words on either side (left and right) of the word WORD 
\w+(?=\sWORD)|(?<=WORD\s)\w+

# Positive lookbehind
# Capture WORD if followed after MATCH  
(?<=MATCH)WORD

# Negative lookbehind
# Capture WORD if WORD doesn't follow MATCH
(?<!MATCH)WORD

# Capture a phrase if it exists in the string and return that phrase
\b(To Be Or Not To Be)\b

# Capture a phrase if it exists in a string (this one does not use word boundaries so it must account for blank spaces)
(To\sBe\sOr\sNot\sTo\sBe)

# Capture any word that appears twice in a string
(\b\w+\b)(?=[\s\S]*\b\1\b)

# Capture any phrase that appears twice in a string
(\b\w+\s+\S+\b)(?=[\s\S]*\b\1\b)

# nbsp spaces
[\u202F\u00A0]

# all ASCII characters	
[\u{0}-\u{7F}]

# all letters but Q and W
[\p{L}--[QW]]

# all non-decimal numbers, plus 0-9
[\p{N}--[\p{Nd}--[0-9]]]	

# all letters in the ASCII range, by subtracting non-letters
[\u{0}-\u{7F}--\P{letter}]

# Greek letters except alpha
[\p{Greek}--\N{GREEK SMALL LETTER ALPHA}]

# all assigned characters except for hex digits (using a broad definition)
[\p{Assigned}--\p{Decimal Digit Number}--[a-fA-Fａ-ｆＡ-Ｆ]]

# either letter or ascii, but not both
[\p{letter}~~\p{ascii}]	
# same as
[[\p{letter}\p{ascii}]--[\p{letter}&&\p{ascii}]]

# matches a single letter that is not a vowel
[a-z&&[^aeiuo]]

# is the same as [4-6] 
# useful for emoji groups
[0-9&&[0-6&&[4-9]]]

[0-9-[0-6-[0-3]]] 
# => first subtracts 0-3 from 0-6
# => yielding [0-9-[4-6]], or [0-37-9]
# => matches any character in the string 0123789

# subtract two classes "sequentially"-ish by adding those classes together first then subtracting 
# To subtract ASCII characters and Greek characters from a class with all Unicode letters
[\p{L}-[\p{IsBasicLatin}\p{IsGreek}]]

# same as \w
[\p{Alphabetic}\p{GC=Mark}\p{GC=Decimal_Number}\p{GC=Connector_Punctuation}]

# remove emojis
(?s)[\p{Basic_Emoji}&&\p{any}]

(?s)[\p{Basic_Emoji}&&\p{any}](?!.*\|)(?=[^\w\d[:punct:]])

# remove emojis that aren't used in a markdown table - needs work
(?s)[\p{Basic_Emoji}&&\p{any}](?!.*\|)

# html equivalent 
[\p{Basic_Emoji}&&\p{any}](?!.*</t[dhr]>)