Regex lookahead ordering -

I am very good with regular expressions, and now I'm trying again to understand to understand And leave the ideas behind. They mostly make sense, but I'm not completely sure how the order affects the results. I see what places look before the expression, and look after the expression. My question is, does this change anything? A recent reply here, SO put a letterhead before the expression which is moving towards my confusion.

When tutorials present the settings, they choose the simplest access case for each one. Then they will use examples such as (? & Lt;! A) b ('no' before 'or') or q (? = U) (' Q followed by 'u' q) This is to avoid avoiding explanation with only distracted details, but it is the tendency to create (or strengthen) the impression that appear in a particular sequence and begin to appear.

Try to look at some more realistic examples. I get a question that involves validating passwords; For example, make sure that the new password is at least six characters long and contains at least one letter and one digit. One way to do this would be:

  ^ (? =. * [AZA-ZED]) (? =. * \ D) [A-Za-z0- 9] {6 ,} $  

character class [A-Za-z0- 9] {6,} can be found with all letters or all digits, so you can use it In order to make sure that there is at least one in each, in this case, you have to look forward to before because the parts after rezux should be able to check the whole string.

For another example, suppose you have to find all the events of the word "there" until it is before the quotation marks. For this obvious regex is (? & Lt;!) [Tt] here \ b , but if you are searching for a large corpus, then it can cause a display problem. As it is written, look at the negative attitude in each position in the regex text, and when it succeeds, it will check all the regex.

Each regex engine has its own strengths and weaknesses, but one thing that is true is that they are faster to find fixed sequences of literal characters than anything else - the more the sequence The better, the better. This means that it can be dramatically fast to see last , even though it means that the word is to match twice:

  [TT] here \ b (? & Lt;! "[TT] here)  

The appointment of the LookAir is that there is no rule;

< / Html> where you place the most importance in every case, put them