To match 3 digit patterns, we can use [0-9][0-9][0-9]. What if we have n digit patterns? We have to write [0-9] n times, but that is a waste of time. Here is when quantifiers come for help.
Limiting repetitions({min, max}): To match n digit patterns, we can simply write [0-9]{n}. Instead of n, by providing minimum and maximum values as-[0-9]{min, max}, we can match a pattern repeating min to max times.
Let's see an example to match all numbers between 1 to 999.
Note: If you don't write the upper bound({min,}), then it basically means, there is no limit for maximum repetitions.
+ quantifier: It is equivalent to {1,}-at least one occurrence.
*quantifier: It is equivalent to {0,}-zero or more occurrences.
? quantifier: It is equivalent to {0,1}, either zero or one occurrence. ? is very useful for optional occurrences in patterns.Let's see an example to match negative and positive numbers.
Now, you may be thinking, what if we want to match characters like *, ?, +, {, } in the text, they are special characters. Check it out in the appendix.
Find out a regex to match positive integers or floating point numbers with exactly two characters after the decimal point.
Answer: \d+(\.\d\d)?
Predict the output of the following regex:
RegEx: [abc]{2,}
Text:
aaa
abc
abbccc
avbcc
Answer:
HTML tag is represented as <tag_name>some text</tag_name>. For example, <title>Regular expression</title>
So, can you figure out an expression that will match both <tag_name> & </tag_name>?
Most of the people will say, it is <.*>. But it gives different result.
So, rather than matching up till first >, it matches the whole tag. So, quantifiers are greedy by default. It is called Greediness!.
To solve this issue, we use ? quantifier and it is called lazy matching. We will discuss it next.
Predict the output of the following regex:
RegEx: (var|let)\s[a-zA-Z0-9_]\w* =\s"?\w+"?;
Text:
var carname = "volvo";
console.log(carname);
let age = 8;
var date = "23-03-2020";
Answer: