Regular expressions are special search strings that are useful for matching complicated patterns. In a regular expression string, many characters have special meanings. For example, there is a special character that means "the beginning of the line".
Source Insight support two different regular expression syntaxes: "Source Insight" and "Perl Compatible". Most places that let you enter a regular expression has a down-down list where you can select the regular expression syntax.
There are regular and "multi-line" versions of each regular expression syntax. The multi-line versions basically match a new-line (end-of-line sequence) with the dot (.) character. So for example, .* would match the whole file. And "start.*end" would find everything from "start" to "end" in the file, even across line breaks.