medit uses regular expressions functionality provided by Glib, which in turn uses PCRE library. See Glib manual for complete description of regular expression syntax.
Regular expression searches in a document text are limited to single lines, unless the
search pattern includes newline character. For example, pattern ".*" will match every
line in the document, pattern ".*\n.*" will match pairs of consecutive lines. This means
that it is mostly impossible to perform searches for text which spawns multiple lines.