Configure Regular Expressions

1. Consider the best practices for using regular expression matches.

   • Use predefined data patterns instead of regular expressions. Use Enterprise DLP predefined data patterns instead of regular expressions where possible. Data patterns are more efficient than regular expressions because the predefined data patterns are tuned for accuracy and the data is validated. For example, if you want to search for social security numbers, use the US Social Security Number (SSN) data pattern instead of a regular expression.
   • Use regular expressions sparingly. Regular expressions can be computationally expensive. If you add a regular expression condition, observe the system for 1 hour for efficient performance. Make sure that the system does not slow down and there are no false positives.
   • Test regular expressions. If you implement regular expression matching, consider using a third-party tool to test the regular expressions before you enable the policy rules. The recommended tool is RegexBuddy. Another good tool for testing your regular expressions is RegExr. If your expression is incorrect, the service can’t match or will match incorrectly.
2. Understand expression terminology.

   Expression Terminology:

   Term	Description
   Literal	A literal is any character you use in a search or matching expression, for example, to find dlp in Enterprise DLP, dlp is a literal string - each character plays a part in the search, it’s literally the string we want to find.
   Metacharacter	A metacharacter is one or more special characters that have a unique meaning and are not used as literals in the search expression, for example, the character < > (caret) is a metacharacter.
   Regular Expression	This term describes the search expression data pattern that you will be using to search in Enterprise DLP.
   Escape Sequence	An escape sequence is a way of indicating that you want to use one of the metacharacters as a literal. In a regular expression an escape sequence involves placing the metacharacter \ (backslash) in front of the metacharacter that you use as a literal, for example, if you want to find (dlp) in Enterprise DLP then use the search expression \(dlp\), and if you want to find \\file in the target string c:\\file then you would need to use the search expression \\\\file (each \ to search for a literal (there are 2) that is preceded by an escape sequence \).

3. Understand expression constructs.

   Enterprise DLP performs Non-deterministic Finite Automation (NFA) based matching with ordered alternation to inspect forwarded traffic against regex.

   NFA-based matching is a regex-directed detection approach that enables Enterprise DLP to explore multiple detection matches simultaneously. When Enterprise DLP encounters a detection choice such as | or *, it picks one path and remember the others using a process called backtracing. This enables Enterprise DLP to return and try alternatives if the initial path fails to match the expression. Palo Alto Networks recommends testing all regex data patterns to ensure efficacy before pushing them to enforcement points in production environments.

   Regular expression constructs:

   Construct	Description
   .	A dot, any single character, except newline (line ending, end of line, or line break) characters.
   \	Escape the next character (the character becomes a normal/literal character.)
   \b	Any word boundary.
   \d	Any digit (0-9.)
   \s	Any white space.
   \W	Any word character (a-z, A-Z, 0-9.)
   \D	Anything other than a digit.
   \S	Anything other than a white space.
   [ ]	Elements inside brackets are a character class (for example, [abc] matches 1 character [a. b. or c.]
   ^	At the beginning of a character class, negates it (for example, [^abc] matches anything except (a, b, or c.)
   $	At the end of a character class, or before the newline at the end.
   +	Following a regular expression means 1 or more (for example, \d+ means 1 more digit.)
   ?	Following a regular expression means 0 or 1 (for example, \d? means 1 or no digit.)
   *	Following a regular expression means any number (for example \d* means 0, 1, or more digits.)
   (?i)	At the beginning of a regular expression makes it case-insensitive (regular expressions are case-sensitive by default.)
   ( )	Groups regular expressions together.
   (?u)	Makes a period ( . ) match to even newline characters.
   |	Means OR (for example, A|B means regular expression A or regular expression B.)

4. Understand expression qualifiers.

   Quantifiers can be used to specify the number or length that part of a pattern should match or repeat. A quantifier will bind to the expression group to its immediate left.

   Regular expression quantifiers:

   Quantifier	Description
   *	Match 0 or more times.
   +	Match 1 or more times.
   ?	Match 1 or 0 times.
   {n}	Match exactly n times.
   {n, }	Match at least n times.
   {n, m}	Match at least n but not more than m times.

5. Enter one regular expression per line, up to 100 lines of expressions.

   There is no limit to the number of regular expressions you can add to a data pattern. Add as many lines of regular expressions as needed.
6. (Weighted expressions only): Assign a regular expression for each line entry between -9999 (lowest importance) to 9999 (highest importance) by entering the regular expression, the delimiter, and the weight score. Enter a weight threshold score of one (1) of more.
7. (Optional) Customize your delimiter.

   By default, the delimiter for all weighted regular expressions is semicolon ( ; ). You can customize your delimiter to copy and paste existing expressions instead of entering them manually. Use a delimiter to specify separate strings of data when configuring regular expressions. For example, you can configure a weighted regular expression using a delimiter to separate the string of text you’re matching from the weight threshold value. If you have large amounts of existing expressions to match, you can customize your delimiter to copy and paste the expressions instead of entering them manually. A delimiter can be any nonalphanumeric, nonbackslash, nonwhitespace character.

   Regular expression delimiters:

   Delimiter	Note
   ;	Semicolon — If the delimiter isn't customized, the semicolon is the default delimiter in Enterprise Data Loss Prevention (E-DLP).
   :	Colon.
   |	Pipe.
   /	Forward Slash — You must escape the delimiter using a backlash ( \ ) if the delimiter needs to match inside the data pattern. If the delimiter appears often inside the pattern, it’s a good idea to choose another delimiter to increase readability.
   +	Plus — Include phrase for matching.
   -	Minus — Ignore phrase for matching.
   #	Hash — Used to denote a number.
   ~	Tilde
   { } Curly	Use brackets to find a range of characters. You don't need to escape bracket-style delimiters when used as meta characters within the pattern, but you must escape bracket-style delimiters when used as literal characters.
   [ ] Square
   ( ) Parenthesis
   < > Caret