Sunday, January 17, 2010

Formatting and documenting your regex

Regular expressions are a very powerful tool for pattern matching, but a complicated
regex can be very difficult for a human to read and to comprehend. That is why,
like any good code, a good regular expression must be well formatted and documented.

Here are some guidelines when formatting and documenting your regex:

1. Keep each line under 80 characters, horizontal scrolling reduces readability.
2. Break long patterns into multiple lines, usually after a space or a line break.
3. Indent bracers to help think in the right scope.
4. Format complicated OR patterns into multiple blocks like a case statement.
5. Comment your regex on what it does, don't just translate it into English.


# Match <BODY
<BODY
# Match any non > char for zero to infinite number of times
[^>]*
# MATCH >
>
Bad example: Comment that translates the regex into English.


# Match the BODY tag
<BODY
# Match any character in the body tag
[^>]*
# Match the end BODY tag
>

Good example: Comment that explains the purpose of the pattern.


(?six-mn:(LabelTextBox)\s+(?<Name>\w+).*(?<Result>\k<Name>\.TextAlign\s*=\s*
((System\.)?Drawing\.)?ContentAlignment\.(?! TopLeftMiddleLeftTopCenterMiddleCenter)\w*)(?!(?<=\k<Name>\.Image.*)(?
=.*\k<Name>\.Image)))

Bad Example: Pray you never have to modify this regex.


(?six-mn:

# Match for Label or TextBox control

# Store name into <name> group

(LabelTextBox)\s+(?<Name>\w+).*

# Match any non-standard TextAlign

# Store any match in Result group for error reporting in CA

(?<Result>

# Match for control's TextAlign Property

\k<Name>\.TextAlign\s*=\s*

# Match for possible namespace

((System\.)?Drawing\.)?ContentAlignment\.

# Match any ContentAlignment that is not in the group

(?!TopLeftMiddleLeftTopCenterMiddleCenter)\w*

)

# Skip any Control that has image on it
(?!
(?<=
\k<Name>\.Image
.*
)
(?=
.*
\k<Name>\.Image
)
)
)
Good Example: Now it make sense!