y allows to perform the search at the given position in the source string.
To grasp the use case of
y flag, and better understand the ways of regexps, let’s explore a practical example.
Writing lexical analyzers is a special area, with its own tools and algorithms, so we don’t go deep in there, but there’s a common task: to read something at the given position.
E.g. we have a code string
let varName = "value", and we need to read the variable name from it, that starts at position
We’ll look for variable name using regexp
- A call to
str.match(/\w+/)will find only the first word in the line (
var). That’s not it.
- We can add the flag
g. But then the call
str.match(/\w+/g)will look for all words in the text, while we need one word at position
4. Again, not what we need.
So, how to search for a regexp exactly at the given position?
Let’s try using method
regexp without flags
y, this method looks only for the first match, it works exactly like
…But if there’s flag
g, then it performs the search in
str, starting from position stored in the
regexp.lastIndex property. And, if it finds a match, then sets
regexp.lastIndex to the index immediately after the match.
In other words,
regexp.lastIndex serves as a starting point for the search, that each
regexp.exec(str) call resets to the new value (“after the last match”). That’s only if there’s
g flag, of course.
So, successive calls to
regexp.exec(str) return matches one after another.
Here’s an example of such calls:
let str = 'let varName'; // Let's find all words in this string let regexp = /\w+/g; alert(regexp.lastIndex); // 0 (initially lastIndex=0) let word1 = regexp.exec(str); alert(word1); // let (1st word) alert(regexp.lastIndex); // 3 (position after the match) let word2 = regexp.exec(str); alert(word2); // varName (2nd word) alert(regexp.lastIndex); // 11 (position after the match) let word3 = regexp.exec(str); alert(word3); // null (no more matches) alert(regexp.lastIndex); // 0 (resets at search end)
We can get all matches in the loop:
Such use of
regexp.exec is an alternative to method
str.matchAll, with a bit more control over the process.
Let’s go back to our task.
We can manually set
4, to start the search from the given position!
Hooray! Problem solved!
We performed a search of
\w+, starting from position
regexp.lastIndex = 4.
The result is correct.
…But wait, not so fast.
Please note: the
regexp.exec call start searching at position
lastIndex and then goes further. If there’s no word at position
lastIndex, but it’s somewhere after it, then it will be found:
For some tasks, including the lexical analysis, that’s just wrong. We need to find a match exactly at the given position at the text, not somewhere after it. And that’s what the flag
y is for.
regexp.exec to search exactly at position
lastIndex, not “starting from” it.
Here’s the same search with flag
As we can see, regexp
/\w+/y doesn’t match at position
3 (unlike the flag
g), but matches at position
Not only that’s what we need, there’s an important performance gain when using flag
Imagine, we have a long text, and there are no matches in it, at all. Then a search with flag
g will go till the end of the text and find nothing, and this will take significantly more time than the search with flag
y, that checks only the exact position.
In tasks like lexical analysis, there are usually many searches at an exact position, to check what we have there. Using flag
y is the key for correct implementations and a good performance.