Making RegEx more readable
Note: this entry
has moved.
Compare the following code statements defining the same
regular expression in .NET:
<![CDATA[static readonly Regex ParameterReference = new Regex(@"(?<empty>\<\>)|\<(?<parameter>[^\<\>]+)\>|(?<open>\<[^\<\>]*(?!\>))",
RegexOptions.Compiled | RegexOptions.IgnorePatternWhitespace);
</open></parameter></empty>]]>
<![CDATA[static readonly Regex ParameterReference = new Regex(@"
# Matches invalid empty brackets #
(?<empty>\<\>)|
# Matches a valid parameter reference #
\<(?<parameter>[^\<\>]+)\>|
# Matches opened brackes that are not properly closed #
(?<open>\<[^\<\>]*(?!\>))",
RegexOptions.Compiled | RegexOptions.IgnorePatternWhitespace);
</open></parameter></empty>]]>
While the former is still understandable for a fairly
regex-aware developer, the later is far more explicit about
the purpose of each part of it. The ability to place
comments inside the expression is enabled by the
RegexOptions.IgnorePatternWhitespace, which is
not used enough by developers. In the case of this pretty
simple expression this may seem unnecessary, but imagine a
regex-based parser that processes (CodeSmith-like) template files:
<![CDATA[static Regex CodeExpression = new Regex(@"
# First match the full directives #
<\#\s*@\s+(?<directive>\w*)(?<attributes>.*?)\#\/>(?:\W*\n)?|
# Match open tag #
(?<open><\#)|
# Match close tag #
(?<close>\#\/>)|
# This is a simple expression that is outputed as-is to output.Write(<output>); #
(?:=)(?<output>.*?)(?<badmultiple>;.*?)?(?=\#\/>)|
# Anything previous or after a code tag #
(?<code>.*?)(?=<\#|\#\/>)|
# Finally, match everything else that is written as-is #
(?<snippet>.*[\r\n]*)",
RegexOptions.IgnorePatternWhitespace | RegexOptions.Compiled | RegexOptions.Singleline);
</snippet></code></badmultiple></output></output></close></open></attributes></directive>]]>
It's pretty obvious that not commenting such complex
expressions makes them almost unreadable except for the guy
who wrote them (and even to him after some time!). Bottom
line: ALWAYS comment
your expressions in-line!!!