regex-howto

• lookahead / lookbehind
  • conclusion
  • detail

lookahead / lookbehind

conclusion

A(?=B)  # positive lookahead
A(?!B)  # negative lookahead

(?<=A)B # positive lookbehind
(?<!A)B # negative lookbehind

So, the = means positive, and the ! means negative, the ? means look, the < means back.

ref:

• lookaround - Regex lookahead, lookbehind and atomic groups - Stack Overflow

detail

I want to match the url in the image tag, but except those online ones.

There are quite a lot ways to help me finish this.

For example, I first matched an img, and extract the url, then test if the url starts with http.

But it's a bit slow, and needs a little more codes. (i.e. not atomic)

I also came up with an 折中的 idea, i.e. just to match those start with '.', which indicates a local url.

However, I want a step further.

I know then it's so-called lookahead or lookbehind.

So I did a quite hard search, and finally I derived my answer from here:

And then I am surprised to find what I wanna to realize can be easily finished via look-ahead based on src=" rather than look-behind based on a ANYTHING.

How 巧妙的 this is!

core ref:

• regex101: build, test, and debug regex

Thanks for YuanHao

• 正则表达式：可读性是什么？能吃吗？ – 小麦冬

• regex - RegExp matching string not starting with my - Stack Overflow

• pcre - RegEx for combining "match everything" and "negative lookahead" - Stack Overflow

ref:

it seems ok but indeed not.

• Regex match string not starting with - Stack Overflow

• Regular Expressions Reference Table of Contents

A detailed tutorial of a professional regex website

• Regex Tutorial - Lookahead and Lookbehind Zero-Length Assertions

• 無聊技術研究: RegExp 應用： lookahead , lookbehind

• Regex to match not start of line - Stack Overflow

• regex - Regular expression for a string that does not start with a sequence - Stack Overflow