javascript - characters - regex match between two strings multiline




Regular Expression to get a string between two strings in Javascript (5)

I have found very similar posts but I can't quite get my regular expression right here.

I am trying to write a regular expression which returns a string which is between two other strings. For example: I want to get the string which resides between the strings "cow" and "milk"

My cow always gives milk

would return

"always gives"

Here is the expression I have pieced together so far:

(?=cow).*(?=milk)

However this returns the string "cow always gives"


Regular Expression to get a string between two strings in Javascript

The most complete solution that will work in the vast majority of cases is using a capturing group with a lazy dot matching pattern. However, a dot . in JS regex does not match line break characters, so, what will work in 100% cases is a [^] or [\s\S]/[\d\D]/[\w\W] constructs.

Scenario 1: Single-line input

cow (.*?) milk

cow is found first, then a space, then any 0+ chars other than line break chars, as few as possible as *? is a lazy quantifier, are captured into Group 1 and then a space with milk must follow (and those are matched and consumed, too).

Scenario 2: Multiline input

cow ([\s\S]*?) milk

Here, cow and a space are matched first, then any 0+ chars as few as possible are matched and captured into Group 1, and then a space with milk are matched.

Scenario 3: Overlapping matches

If you have a string like >>>15 text>>>67 text2>>> and you need to get 2 matches in-between >>>+number+whitespace and >>>, you can't use />>>\d+\s(.*?)>>>/g as this will only find 1 match due to the fact the >>> before 67 is already consumed upon finding the first match. You may use a positive lookahead to check for the text presence without actually "gobbling" it (i.e. appending to the match):

/>>>\d+\s(.*?)(?=>>>)/g

See the online regex demo yielding text1 and text2 as Group 1 contents found.

Also see How to get all possible overlapping matches for a string.

Performance considerations

Lazy dot matching pattern (.*?) inside regex patterns may slow down script execution if very long input is given. In many cases, unroll-the-loop technique helps to a greater extent. Trying to grab all between cow and milk from "Their\ncow\ngives\nmore\nmilk", we see that we just need to match all lines that do not start with milk, thus, instead of cow\n([\s\S]*?)\nmilk we can use:

/cow\n(.*(?:\n(?!milk$).*)*)\nmilk/gm

See the regex demo (if there can be \r\n, use /cow\r?\n(.*(?:\r?\n(?!milk$).*)*)\r?\nmilk/gm). With this small test string, the performance gain is negligible, but with very large text, you will feel the difference (especially if the lines are long and line breaks are not very numerous).

Sample regex usage in JavaScript:

//Single/First match expected: use no global modifier and access match[1]
console.log("My cow always gives milk".match(/cow (.*?) milk/)[1]);
// Multiple matches: get multiple matches with a global modifier and 
// trim the results if length of leading/trailing delimiters is known
var s = "My cow always gives milk, thier cow also gives milk";
console.log(s.match(/cow (.*?) milk/g).map(function(x) {return x.substr(4,x.length-9);}));
//or use RegExp#exec inside a loop to collect all the Group 1 contents
var result = [], m, rx = /cow (.*?) milk/g;
while ((m=rx.exec(s)) !== null) {
  result.push(m[1]);
}
console.log(result);

A lookahead (that (?= part) does not consume any input. It is a zero-width assertion (as are boundary checks and lookbehinds).

You want a regular match here, to consume the cow portion. To capture the portion in between, you use a capturing group (just put the portion of pattern you want to capture inside parenthesis):

cow(.*)milk

No lookaheads are needed at all.


I was able to get what I needed using Martinho Fernandes' solution below. The code is:

var test = "My cow always gives milk";

var testRE = test.match("cow(.*)milk"); alert(testRE[1]);

You'll notice that I am alerting the testRE variable as an array. This is because testRE is returning as an array, for some reason. The output from:

My cow always gives milk

Changes into:

always gives

The chosen answer didn't work for me...hmm...

Just add space after cow and/or before milk to trim spaces from " always gives "

/(?<=cow ).*(?= milk)/


  • You need capture the .*
  • You can (but don't have to) make the .* nongreedy
  • There's really no need for the lookahead.

    > /cow(.*?)milk/i.exec('My cow always gives milk');
    ["cow always gives milk", " always gives "]
    




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