topic Re: What does (?m) mean at the beginning of a regex in Splunk Search

What does (?m) mean at the beginning of a regex

Justin_Grant — Fri, 11 Dec 2009 04:15:35 GMT

I saw this in \etc\system\README\transforms.conf.example:

REGEX = (?m)^(.*)SessionId=\w+(\w{4}[&"].*)$

What does the (?m) mean before the caret? Is this really matching 0 or 1 "m" characters at the end of the previous line, or does it have some special meaning?

Re: What does (?m) mean at the beginning of a regex

Johnvey — Fri, 11 Dec 2009 05:08:07 GMT

It declares the regex to read multiline data, i.e., don't stop the regex on a line break.

The (?<option_flag>) construct allows you to set various matching properties like case-insensitivity, multiline, greedy, etc. See http://www.regextester.com/pregsyntax.html for more info.

Re: What does (?m) mean at the beginning of a regex

gkanapathy — Fri, 15 Jan 2010 02:51:04 GMT

In general, all Splunk regexes use the PCRE flavor of regex, which is substantially the same regex syntax as Perl, Python, PHP, but significantly different from grep (or egrep).

Re: What does (?m) mean at the beginning of a regex

Jason — Tue, 08 Oct 2013 04:27:43 GMT

More importantly, this "multiline mode" means that ^ and $ match the beginning and end (respectively) of each line, not the beginning and end of the entire string. This is important in multiline events, in case you want to find an item at the beginning of a line somewhere in your event.

Re: What does (?m) mean at the beginning of a regex

landen99 — Fri, 17 Jul 2015 20:50:51 GMT

And just as importantly, rex will match against the entire event without (?m) even if there are line breaks. With max_match=0, rex will even match on the pattern multiple times in the same event thus creating a multi-value field.