Regex — Remember Lookbehind Sun and Lookahead Moon

REGEX.md

Regex — Remember Lookbehind Sun and Lookahead Moon

How to remember lookbehind and lookahead regular expression.

Now I show some pictures for my expression.

There is regular expression

You know caret means start

And dollar means end

If a caret is day

Then a dollar is night

Time advances from day to night

Because regex match advances from left to right means start to end.

Take a Time

You know regex lookahead and lookbehind?

I will express by same way about 4.

Let's be friend with mysterious marks

Umm take a Time second time

Behind means like Start and Head means like End

Time advances in same world

In the first place we can ignore question mark

Because it is common mark of starting special regex syntax.

Equal means Yes

Bang means No

"Positive Lookbehind" has "An toward past" and says "Yes".

"Negative Lookbehind" has a "An toward past" and says "No".

"Positive Lookahead" has no mark about time advance

But now imagine omitted mark toward Head.

Because time advances toward head is very natural.

So we do not have to express about it.

"Pegative Lookahead" has omitted mark toward head and says "Yes".

"Negative Lookahead" has omitted mark toward head and says "No".

Examples

Replace "B" with "X" in "ABC".

Positive Lookbehind

$ echo "ABC" | perl -pe 's/(?<=A)B/X/g'
AXC

Negative Lookbehind

$ echo "ABC" | perl -pe 's/(?<!A)B/X/g'
ABC

Positive Lookhead

$ echo "ABC" | perl -pe 's/B(?=C)/X/g'
AXC

Negative Lookhead

$ echo "ABC" | perl -pe 's/B(?!C)/X/g'
ABC

Versions

• bash-4.4
• perl 5, version 28