How to write custom regular expression for url(custom url)

Question

My application accepts the following type of URL

Example:

ABa12C.constant.com or

http://ABa12C.constant.com or

https://ABa12C.constant.com

(Optional)Starts with http:// or https:// or nothing.
Should not start with integer(1aAb4.constant.com is invalid).
In place of com it can be any like(in,uar,sr etc) but no integers.

My regular expression: [http://a-zA-Z0-9]{1,20}.constant.[a-zA-Z]{1,5}

But how to make http:// optional and can be used only at beginning and cannot be integer at beginning.

Looks like you misuse character classes. What are you trying to do with [http://a-zA-Z0-9]{1,20}? Try ^(https?:\/\/)?[^0-9][a-zA-Z0-9]{0,19}\.constant\.[a-zA-Z]{1,5}$ — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Jul 18, 2016 at 9:28
^ means checks at the start of string and ? means optional. But [^0-9] makes integer compulsory but i don't want integer at beginning. — Nithin B
– Nithin B, Commented Jul 18, 2016 at 9:50
You have not answered the question: what are you trying to achieve with [http://a-zA-Z0-9]{1,20}? There is no any length restriction requirement listed in your question. Note that [^0-9] makes a non-digit compulsory. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Jul 18, 2016 at 9:57
Yes its working. So here ^ acting as start of string and also as exclude [0-9]. I mean does ^ character have two meanings in different locations. — Nithin B
– Nithin B, Commented Jul 18, 2016 at 11:27
[^0-9] matches any character other than a digit. It matches ~, \n, *, a, _, space. A lot of things. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Jul 18, 2016 at 11:42

Wiktor Stribiżew · Accepted Answer · 2016-07-18 10:15:40Z

1

Your pattern has no anchors, and the initial subpattern is a character class [http://a-zA-Z0-9]{1,20} that matches 1 to 20 chars from the class, either h or t, p, :, /, a-z, A-Z, 0-9 while you need to match http:// as a sequence.

I suggest

^(https?:\/\/)?[a-zA-Z][a-zA-Z0-9]{0,19}\.constant\.[a-zA-Z]{1,5}$

See the regex demo

Explanation:

^ - start of string
(https?:\/\/)? - an optional sequence of http:// or https://
[a-zA-Z] - an ASCII letter
[a-zA-Z0-9]{0,19} - 0 to 19 alphanumeric characters (the length restriction can be adjusted by you)
\.constant\. - a constant substring .constant.
[a-zA-Z]{1,5} - 1 to 5 ASCII letters
$ - end of string.

answered Jul 18, 2016 at 10:15

Wiktor Stribiżew

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Wiktor Stribiżew Over a year ago

Glad it worked for you. Please also consider upvoting if my answer proved helpful to you (see How to upvote on Stack Overflow?).

CinCout · Accepted Answer · 2016-07-18 10:19:44Z

0

The following regex does the trick:

(?:https?:\/\/)?[^0-9]\S*\.[^0-9]\S* demo

Note that a non-capturing group (?:) is used for the protocol.

answered Jul 18, 2016 at 10:19

CinCout

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Wiktor Stribiżew Over a year ago

It does not matter much if the group is capturing or not, but the [^0-9] will allow any symbol other than digits, while it seems OP needs only alphanumeric values in the input string.

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