By inserting this regex in a programming language to test it against Codeforces 1800A you'll solve the task

style img

Using the twitter API, take a tweet object and find & link #tags to embed in HTML

Capture alternative items matching end of lines country codes

Matches all valid, current UK Postcodes, including Girobank and non-geographic postcodes, irrespective of whether they contain a space. It does not include overseas territories. Adapted from the BS7666 postcode rules at http://www.cabinetoffice.gov.uk/govtalk/schemasstandards/e-gif/datastandards/add...

match bibtex entries

This regex matches java line end comments started with "//" and ignores such ones that are within String literals. (NO SUFFICIENT TESTING WAS DONE TO GUARANTEE 100% SAFETY)

Looks for string formated like an IP address.

email validation based on the restrictions for entering email adresses into a mongo db email field.

Validates all possible international phone number formats. Icludes almoust all of the international call prefixes, all of the differing lenghts of country codes. The parts of number will always be recognised correctly. Additionally you could add limitations of the phone number's lenght

Matches all ips that points to localhost like, optionally capturing port number: 127.X.X.X and 127.X.X.X: