MatthewMDavis
String.split on a sub-string within a pattern?
I’m trying all sorts of things with no luck, so I thought I’d ask for some help.
I want to split a string anytime there’s a “b” in “abc”, but not in " b ". I.e., when a “b” is surrounded by whitespace, I don’t want it to be a splitting character, but an element in the array returned by the split.
String.split("abcde", regex) == ["a", "cde"]
String.split("a b cde", regex) == ["a", "b", "cde"]
So I need (I think) a way to craft a regex that looks for abc, but only splits on the b part.
Marked As Solved
jwarlander
Regex Negative Lookbehind is your friend in cases like this:
iex(24)> String.split("abcde", ~r/((?<!\s)b(?!\s)|\s)/)
["a", "cde"]
iex(25)> String.split("a b cde", ~r/((?<!\s)b(?!\s)|\s)/)
["a", "b", "cde"]
iex(26)> String.split("ab cde", ~r/((?<!\s)b(?!\s)|\s)/)
["ab", "cde"]
iex(27)> String.split("a bcde", ~r/((?<!\s)b(?!\s)|\s)/)
["a", "bcde"]
The above pattern will match a b for splitting, only if it’s entirely surrounded by non-whitespace, eg. it’s not preceded by whitespace ((?<!\s)) and it’s not followed by whitespace ((?!\s)). It’ll also split on standalone whitespace ((...|\s)).
Depending on your exact needs you may want to adjust the pattern, of course.
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JEG2
I think the pattern can be even simpler. Can you show some sample inputs and what you want to see as outputs?
swelham
I read this blog series(sorry it’s in python) last year that I think will help. It’s about building a simple interpreter and it starts off with simple maths and then onto supporting order of operation. Also requires no regex 
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