[Bro-Dev] case-insensitive patterns
vern at corelight.com
Fri Jun 29 12:00:30 PDT 2018
Once I wound up monkeying around with the internals of the pattern-matching
code (to fix leaks, because Johanna [correctly] pushed back on adding the
&/| operators for general use if they leaked, which an old ticket indicated
they would) ... I thought what-the-heck, it's time for supporting
This turned out to be tricky to implement, as I gleaned from talking with
Seth about an approach he had tried a while back but abandoned. But I now
have it working. Here's the blurb from the NEWS entry in the
- You can now specify that a pattern matches in a case-insensitive
fashion by adding 'i' to the end of its specification. So for example
/fOO/i == "Foo" yields T, as does /fOO/i in "xFoObar". Characters
enclosed in quotes however keep their casing, so /"fOO"/i in "xFoObar"
yields F, though it yields T for "xfOObar".
You can achieve the same functionality for a subpattern enclosed in
parentheses by adding "+i" to the open parenthesis, optionally followed
by whitespace. So for example "/foo|(+i bar)/" will match "BaR", but
For both ways of specifying case-insensitivity, characters enclosed in
double quotes maintain their case-sensitivity. So for example /"foo"/i
will not match "Foo", but it will match "foo".
The funky (+i ...) syntax isn't meant for general user consumption (though
it's okay if a user wants to use it directly), but rather is how I implemented
/pattern/i functionality. Basically, /pattern/i turns into /(+i pattern)/.
That switch is necessary because the robust way to implement case-insensitive
patterns, such that they can be composed with the & and | operators and
behave as expected, is to modify the parsing of REs to turn any instance
of a letter into a character class (so that /foo/ becomes /[Ff][Oo]Oo]/,
just like people have been doing by hand for years), and also to modify
the parsing of character classes. That requires alerting the RE scanner
that it's doing a case-insensitive (sub)pattern, which in turn requires
a prefix operator that specifies case-insensitivity.
Let me know if you have any concerns. Otherwise, I'll tee this up
for merging early next week.
More information about the bro-dev