Regex to Match Date

Regex to validate date formats dd/mm/YYYY, dd-mm-YYYY, dd.mm.YYYY, dd mmm YYYY, dd-mmm-YYYY, dd/mmm/YYYY, dd.mmm.YYYY with Leap Year Support

The regex you pasted does not validate leap years correctly, but there is one that does in the same post.
I modified it to take dd/mm/yyyy, dd-mm-yyyy or dd.mm.yyyy.

^(?:(?:31(\/|-|\.)(?:0?[13578]|1[02]))\1|(?:(?:29|30)(\/|-|\.)(?:0?[13-9]|1[0-2])\2))(?:(?:1[6-9]|[2-9]\d)?\d{2})$|^(?:29(\/|-|\.)0?2\3(?:(?:(?:1[6-9]|[2-9]\d)?(?:0[48]|[2468][048]|[13579][26])|(?:(?:16|[2468][048]|[3579][26])00))))$|^(?:0?[1-9]|1\d|2[0-8])(\/|-|\.)(?:(?:0?[1-9])|(?:1[0-2]))\4(?:(?:1[6-9]|[2-9]\d)?\d{2})$

I tested it a bit in the link Arun provided in his answer and also here and it seems to work.

Edit February 14th 2019: I've removed a comma that was in the regex which allowed dates like 29-0,-11

Regex to match date formats DD-MM-YYYY and DD/MM/YYYY

This pattern will work for your conditon. Strictly not allowed month to date and vice versa.

^([0]?[1-9]|[1|2][0-9]|[3][0|1])[./-]([0]?[1-9]|[1][0-2])[./-]([0-9]{4}|[0-9]{2})$

This pattern will match these formats: (Date, Month, Year)

25.04.2017  
02.04.2017  
2.4.2017  

25/04/2017  
5/12/2017  
15/2/2017  

25-04-2017  
6-10-2017  
16-5-2017  

Regular expression to match a date range

Wouldn't it be easier to parse the strings to DateTimes and comparing those?

RegEx for matching dates (Month Day, Year OR m/d/yy)

Your expression seems to work fine, both of them. If you wish to capture anything before or after your target output, you can simply add two boundaries in the left and right, which would do that for you. For example, please look at this test:

(.*)(((1[0-2]|0?[1-9])\/(3[01]|[12][0-9]|0?[1-9])\/(?:[0-9]{2})?[0-9]{2})|((Jan(uary)?|Feb(ruary)?|Mar(ch)?|Apr(il)?|May|Jun(e)?|Jul(y)?|Aug(ust)?|Sep(tember)?|Oct(ober)?|Nov(ember)?|Dec(ember)?)\s+\d{1,2},\s+\d{4}))(.*)

where you can for instance add two groups similar to (.*) and wrap your original expression in one group which will do so.

RegEx Descriptive Graph

The graph visualize how your expression works and you might want to test other expressions in this link:

C# Test

using System;
using System.Text.RegularExpressions;

public class Example
{
    public static void Main()
    {
        string pattern = @"(.*)(((1[0-2]|0?[1-9])\/(3[01]|[12][0-9]|0?[1-9])\/(?:[0-9]{2})?[0-9]{2})|((Jan(uary)?|Feb(ruary)?|Mar(ch)?|Apr(il)?|May|Jun(e)?|Jul(y)?|Aug(ust)?|Sep(tember)?|Oct(ober)?|Nov(ember)?|Dec(ember)?)\s+\d{1,2},\s+\d{4}))(.*)";
        string input = @"Paid on Jan 1, 2019 And anything else that you wish to have after
Paid on 1/1/2019 And anything else that you wish to have after";
        RegexOptions options = RegexOptions.Multiline;

        foreach (Match m in Regex.Matches(input, pattern, options))
        {
            Console.WriteLine("'{0}' found at index {1}.", m.Value, m.Index);
        }
    }
}

JavaScript Demo

This JavaScript demo shows that your expression works:

const regex = /(.*)(((1[0-2]|0?[1-9])\/(3[01]|[12][0-9]|0?[1-9])\/(?:[0-9]{2})?[0-9]{2})|((Jan(uary)?|Feb(ruary)?|Mar(ch)?|Apr(il)?|May|Jun(e)?|Jul(y)?|Aug(ust)?|Sep(tember)?|Oct(ober)?|Nov(ember)?|Dec(ember)?)\s+\d{1,2},\s+\d{4}))(.*)/gm;const str = `Paid on Jan 1, 2019 And anything else that you wish to have afterPaid on 1/1/2019 And anything else that you wish to have after`;const subst = `\nGroup 1: $1 \nGroup 2: $2 \nGroup 3: $3 \nGroup 4: $4 `;
// The substituted value will be contained in the result variableconst result = str.replace(regex, subst);
console.log('Substitution result: ', result);

[0-9]{2}/[0-9]{2}/[0-9]{4}

(?:([0-9]+)/([0-9]+)/([0-9]+)|((0?[1-9]|1[0-2])-(0?[1-9]|[12]\d|3[01])-(\d{4}|\d{2}))|\w+\s\d{2},\s\d{4}|(?i)\b(Jan(?:uary|.)?|Feb(?:ruary|.)?|Mar(?:ch|.)?|Apr(?:il|.)?|May|Jun(?:e|.)?|Jul(?:y|.)?|Aug(?:ust|.)?|Sep(?:tember|.)?|Oct(?:ober|.)?|Nov(?:ember|.)?|Dec(?:ember|.)?)( ,?[ ]|-(?:0?[1-9]|[1-2][0-9]|3[01])-)(\d{4}))(?=\h+More\b)

\/tmp\/(2019060[6-9]|201906[1-3][0-9]|20190[7-9][0-9]{2})