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Last Updated: May 15, 2016
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· adrian2112
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Lookahead and Lookbehind regex

Sometimes we need to look if a string matches or contains a certain pattern and that's what regular expressions (regex) are for.
But sometimes we have the condition that this pattern is preceded or followed by another certain pattern.
We certainly can do that as easy as adding this other pattern to the one we are looking for

Pattern looked for: [0-9]*
Preceded by: [a-z]
Join and profit: [a-z][0-9]*

But what happens if we need this condition to be matched but we want to get the string that matched the pattern without the conditioning pattern

a = "k47"

# match all numbers preceded by a letter
a.match(/[a-z][0-9]*/)[0]

# but we just want the number :(
> "k47"

Introducing lookahead and lookbehind regex


lookahead

(?=regex)

Meaning: followed by the expression regex but without including it in the match

ej.

a yolo followed by a lo but without including it in the match

http://rubular.com/?regex=yolo(?=lo)&test=yolo%20yololo

input: yolo yololo

regex: yolo(?=lo)

result: yolo yololo


(?!regex)

Meaning: not followed by the expression regex

ej.

a yolo not followed by a lo

http://rubular.com/?regex=yolo(?!lo)&test=yolo%20yololo

input: yolo yololo

regex: yolo(?!lo)

result: yolo yololo


lookbehind

(?<=regex)

Meaning: preceded by the expression regex but without including it in the match

ej.

a lol preceded by a yo but without including it in the match

http://rubular.com/?regex=(?%3C=yo)lol&test=lol%20yololo

input: lol yololo

regex: (?<=yo)lol

result: lol yo​lolo


(?<!regex)

Meaning: not preceded by the expression regex

ej.

a lol not preceded by a yo

http://rubular.com/?regex=(?%3C!yo)lol&test=lol%20yololo

input: lol yololo

regex: (?<!yo)lol

result: lol yololo


Ruby example

a.gsub(/(?<=:)[0-9]+/, '"\0"')

Surround any number preceded by a : between "

a = "{a:123,c:12345}"

a.gsub(/(?<=:)[0-9]+/, '"\0"')

> "{a:\"123\",c:\"12345\"}"

the regex syntax can change depending on the language

http://wiki.icalialabs.com/articles/1

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