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A simple regular expression example

A simple regular expression example

Asfaq Leeon's photo
Asfaq Leeon
·

3 min read

Once upon a time, simple regex patterns are really hard to understand to me. But now-a-days, I can write simple regex patterns comfortably. Few weeks ago, I also have written a complex one. Today I'm going to share how I approach a regex problem.

Today I'm going to build a regular expression pattern to match bangladeshi mobile phone numbers. I'll test with different numbers and also extract to a list in JavaScript.

N.B: The regex pattern I've shown here can be different from you. There are many ways to write regex.

Some valid Bangladeshi phone numbers

Before finding some patterns, let's look some generated random valid phone numbers. Feel free to ignore this.

  • 01600000000

  • +8801611111111

  • 8801622222222

  • 1633333333

  • 01312312312

  • 01501201201

  • 01777777777

  • 01800000000

  • 01912341234

Rules

Here are the rules for any bangladeshi mobile phone number.

  • Country code +880

  • with +, the length is 15

  • without + but starts with 880, the length is 14

  • with 0, the length is 11

  • without 0, the length is 10

  • after 0, start with 1

  • after 1, only 3,4,5,6,7,8,9 till 2024

  • after +, only digits

For simplicity, let's skip 2 rules

  • Country code +880

  • start with 0 not 1

Regex step by step

  • [0] for matching 0

  • [1] for matching 1

  • [356789] for matching only any of 3,5,6,7,8,9

  • \d for matching any digits after

  • \d{8} for only 8 next digits only.

  • there is a case if there are more than 8 digits, we put $ for end of string.

  • But some invalid cases will also be captured like

    • +8801611111111

    • 8801622222222

      So, we need to start from 0 not any other digits. We can easily do this using negative lookbehind (?<!\d). So, the final regex is

(?<!\d)[0][1][356789]\d{8}$

N.B: This is a possible way. There are different ways to build patterns.

Testing regex in online

There are many online tools for building, matching or testing regex. Currently I'm using regex101

In this website's middle, put your expression in REGULAR EXPRESSION section and test data in TEST STRING section.

In left side, there are 3 valuable sections

  • EXPLANATION: This section describes the given expression step by step.

  • MATCH INFORMATION: This sction shows the result

  • QUICK REFERENCE: If we need any regex pattern reference, this is the place.

Play with others rules

Now let's try to add some other rules.

  • A valid number can start with +8801 or 8801 or 01 or even only 1

  • here 1 is common and only 1 is valid then use quantifier like this 1{1}

  • Now for +880, 880, 0 we can use the alternate(OR) operator like this (\+880|880|0)

  • Now this condition (\+880|880|0) can be present or not so, we can use this ? operator.

So, a possible pattern can be

(\+880|880|0)?1{1}[356789]\d{8}$

Capturing groups

Now if we want to extract the numbers then we can use single braces ()

((?<!\d)[0][1][356789]\d{8}$)

Let's write some code and extract some phone numbers

const expression = /((?<!\d)[0][1][356789]\d{8}$)/gm

const testString = 
`* 0167561999789900
* 5896988016769696
* 112589696
* 01475619997

* 01600000000
* +8801611111111
* 8801622222222
* 1238801622222222
* 1633333333
* 01312312312
* 46467401600000000
* 345589+8801611111111
* 01501201201
* 01777777777
* 01800000000
* 01912341234`

const phoneNumberList = testString.match(expression)

phoneNumberList.forEach(phoneNumber => console.log(`Extracted number: ${phoneNumber}\n`))

Like bash, in JavaScript, the regular expression should be written inside slashes(\). After second slash there is gm. Here g means global and m means multiline. Let's keep those explanation for another day. If you run this code, the output may like this

Output:

Extracted number: 01600000000

Extracted number: 01312312312

Extracted number: 01501201201

Extracted number: 01777777777

Extracted number: 01800000000

Extracted number: 01912341234

Conclusion

I hope you enjoyed this regex example. If you have any doubt please let me know in the comment section. If you like this content don't forget to give a thumb. Happy coding.

RegexRegular Expressionspattern-matching