Regex - Match a Pattern Before a Character

Question

Hi, I'm currently building a toy assembler in c# (going through The Elements Of Computing Systems book).

I need to match a very simple pattern, I thought this would be a good time to learn some regex but I'm struggling!

In the following examples I'd just like to match the letters before the '='

M=A

D=M

MD=A

A=D

AD=M

AMD=A

I've come up with the following:

([A-Z]{1,3})=

However this also matches the '=' which I don't want.

I also tried:

([A-Z^\=]{1,3})=

But I still have the same problem - it a matches the '=' sign as well.

I'm using this site to test my regexes.

Any help would be really appreciated. Thank you in advance.

Answer 1

You need a positive lookahead assertion:

([A-Z]{1,3})(?==)

Answer 2

What you want is called a zero-width, lookahead assertion. You do:

(Match this and capture)(?=before this)

In your case, this would be:

([A-Z^]{1,3})(?==)

Answer 3

The following will group everything before the "=" and everything after.

([^=]*)=([^=]*)

it reads something like this:

match any amount of characters thats not a "=", followed by a "=", then any amount of characters thats not a "=".

Answer 4

Just to thank you guys. Saved me a lot of time.

Answer 5

You can also put the equals sign in a non-capturing parans with (?: ... )

([ADM]{1,3})(?:=)

It's been a bit since I did this chapter of the book but I think that since you need both parts of the expression anyway, I did a split on the = resulting in myArray[0] == M, myArray[1] == A