Valid PHP variable name as defined in the manual at http://php.net/manual/en/language.variables.basics.php

Check for true or false street address format

A quite neat regular expression for URLs, e-mails..., I found on Matthew O'Riordan's blog.

1-15 alphanumeric login name

By Using this string you can just remove ascii special characters from a unicode string characters like ♥♥♥♥ ▓▒ and other non unicode letters.

global modifier

Use this regex to select all characters up to the "" bracket. It works across multiple lines. You can get an email list with only the IDs. This is the pattern it matches: Alpha Person Step 1 (.*) [$ Step 2 [$.$] (.*) gives you aperson@whatever.org...

Matches any valid JSON object, matches recursively. Passes all JSON test cases I found.

Finds the outermost pair of parentheses. Compatible with any amount of nesting

Handles commands like in CMD

Match nth occurence of the patterns in the inner parenthesis. Put nth that you want to match in the curly braces (here, {3}).

Detects valid Roman numerals (up to 3999), and splits into 4 capture groups: thousands, hundreds, tens, and units.

Text output produced by reading the QR code present on patient stickers created by Epic (R) medical record software