Positive lookahead with javascript regex – Even if we have a good project plan and a logical concept, we will spend the majority of our time correcting errors abaout javascript and regex. Furthermore, our application can run without obvious errors with JavaScript, we must use various ways to ensure that everything is operating properly. In general, there are two types of errors that you’ll encounter while doing something wrong in code: Syntax Errors and Logic Errors. To make bug fixing easier, every JavaScript error is captured with a full stack trace and the specific line of source code marked. To assist you in resolving the JavaScript error, look at the discuss below to fix problem about Positive lookahead with javascript regex.
Problem :
I have been messed up with regex.. I found it difficult for me..I have seen a code like:
function myFunction() {
var str = "Is this all there is";
var patt1 = /is(?= all)/;
var result = str.match(patt1);
document.getElementById("demo").innerHTML = result;
}
When i run this code it gave me the output is.
But when i add like /is(?=there)/ it didnt output anything. I am new to regular expression ..hope you guys can help in in understanding positive lookahead in regex..I have followed many tutorials it didnt helped me.
Hope you guys can help me out. Thanks!
Solution :
The regex is(?= all) matches the letters is, but only if they are immediately followed by the letters all
Likewise, the regex is(?=there) matches the letters is, but only if they are immediately followed by the letters there
If you combined the two in is(?= all)(?=there), you are trying to match the letters is, but only if they are immediately followed both by the letters all AND the letters there at the same time… which is not possible.
If you want to match the letters is, but only if they are immediately followed either by the letters all or the letters there, then you can use:
is(?= all|there)
If, on the other hand, you want to match the letters is, but only if they are immediately followed by the letters all there, then you can just use:
is(?= all there)
What if I want is to be followed by all and there, but anywhere in the string?
Then you can use something like is(?=.* all)(?=.*there)
The key to understanding lookahead
The key to lookarounds is to understand that the lookahead is an assertion that checks that something follows, or precedes at a specific position in the string. That is why I bolded immediately. The following article should dispel any confusion.
Reference
The Positive Lookahead fails for the fact that there does not immediately follow is.
is(?=there) # matches is when immediately followed by there
To match is if there follows somewhere in the string, you would do:
is(?=.*there)
Explanation:
is # 'is'
(?= # look ahead to see if there is:
.* # any character except n (0 or more times)
there # 'there'
) # end of look-ahead
See Demo
A detailed tutorial I recommend: How to use Lookaheads and Lookbehinds in your Regular Expressions