Capture text after multiple optional strings into named group with Regex

Question

I am trying to extract multiple strings using different patterns from one long string. Here is an example of the input string:

[Update 2]Number of students: 5[New]Break at 1:45 pm\nStudents involved are: John, Joseph, Maria\nLunch at 2:00pm\nActivities remaining: long jump, shuffle

There are three prefixes which are used to extract the data after it: 'Students involved are:', 'Activities remaining:', 'Number of students:'. I managed to extract the above into a named group using the following Regex:

let pattern = /(?<=Number of students: )(?<number>[^\n]+).*?(?<=Students involved are: )(?<students>[^\n]+).*?(?<=Activities remaining: )(?<activities>[^\n]+)/gms
let match = pattern.exec(s)
const num = match.number;
const activities = match.activities;

The above works. However, I run into an issue when there is one of the strings missing. All the three prefixes I am searching for are optional. How can I modify the regex to handle optional patterns? Or is there a better way of accomplishing this? Thanks!

Answer 1

I'm not sure you need look behind assertions for that use case...

To answer your question you can wrap your individual patterns inside non-capturing groups followed by a question mark:

const r = /(?:Desc.1:\s*(?<tag1>.*?))?(?:Descr.2:\s*(?<tag2>.*?))?(?:Desc.3:\s*(?<tag3>.*?))?/

If the values can come in any order, you can use a global match and a disjunction:

const r = /x(?<tag1>.*)|y(?<tag2>.*)|z(?<tag3>.*)/g

for (const {groups: {tag1, tag2, tag3}} of source.matchAll(r)) {
 ...
}

You can see it in action here

Also, FYI, the flags you're using don't make a lot of sense to me:

• g is useful to match several times (e.g. with "".matchAll(/regexp/g), but it is useless otherwise)
• m makes ^ and $ assertions match the start and end of lines on top of their usual duty, but you're not using them

Answer 2

Here's my attempt:

"^\[[^\]]+\](Number of students: )*(?<number>[^\n]+)\\n(Students involved are: )*(?<students>[^\n]+)\\n(Activities remaining: )*(?<activities>[^\n]+)"

The differences between mine and yours are the following:

• added ^\[[^\]]+\] at the beginning to match [<any characters>]
• added * over the optional parts of your string
• added \\n between the paired three parts

I've tested this regex with these two examples:

• [Update 2]Number of students: 5[New]Break at 1:45 pm\nStudents involved are: John, Joseph, Maria\nLunch at 2:00pm\nActivities remaining: long jump, shuffle
• [Update 2]5[New]Break at 1:45 pm\nJohn, Joseph, Maria\nLunch at 2:00pm\nlong jump, shuffle

Does it work for you?

ps. for any attempt to increased efficiency, more samples of pattern matching are needed