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\chapter{Exercise 16: Search And Replace}
Now that you know about groupings I'm going to show you how to do search and
replace. If you remember from the last exercise you can surround a regex with
parenthesis and it'll create a capture. In this exercise I'll show you how
to use that to extract parts of a string and then replace them, as well
as just doing simple replacements.
To do this exercise I'll make a corpus text that has an IP address, some
numbers, and a web URL that's wrong. Then I'll play with them.
<< d['code/ex16.txt'] >>
I want to make a search/replace that replaces only IP addresses with
"X.X.X.X" so that it is sanitized. I want another regex that will fix this
URL so that it has \emph{https} and is a correct URL not just a file.
<< d['code/ex16.regex'] >>
The first line I'm doing the IP address replace wrong, which you can see
when we run it as it replace *all* numbers with X. The second regex is
more correct and replaces only the IP address. From these two you can see
the form for a replace in Regetron is:
\item !rep to start a \emph{rep}lace
\item A \verb|/| to set the bound of the \emph{search} part.
\item A regex for searching
\item Another \verb|/| to end the search part.
\item A string for the \emph{replace} part.
\item An ending \verb|/| to end the whole thing.
That means the form is \verb|!rep /SEARCH/REPLACE/| but there's a great modification
to this that comes in handy. The \verb|/| can be any character, which solves the
problem of doing a search/replace inside a string that has a \verb|/| character
in it already. I demo this in the third regex by using \verb|!rep ,SEARCH,REPLACE,|
The final thing to look at is the third regex has a grouping (aka capture), which
you know about, but then I do \verb|\1|. What this does it is grabs the first
group that was matched (that's the 1) and \emph{inserts} it right there in the
\section{What You Should See}
Pay special close attention to this and make sure you understand how the replacements
are working:
\begin{code}{ex16 Output}
<< d['code/ex16.regex|regetron']['ex16.txt'] >>
The most important part is the last regex and the use of the \verb|\1| to grab
the group from the search part and put it in the replace part. In this case
the \verb|(.+?)| is matching the \verb|index.html| part, and then the \verb|\1|
is placing it in the corrected link inside the replace. One more thing to
realize is if you had 3 groups in the search, then you'd have access to \verb|\2|
and \verb|\3| as well.
\section{Extra Credit}
\item If you use Vim or Emacs then you have access to this as a search and
replace operation. In vim try loading the corpus text and typing
\verb|:%s ,http://\(.*\)/,\1,|
which has a slightly different search part so pay attention. Notice
I have to escape the parens with \verb|\(|.
\item Write a search replace that replace animals with just "dog", because
dogs are better.
\item Write a regex that takes a URL with a file path and keeps everything
but the file, replacing it with \verb|/index.html|.
\item Why did I use \verb|.+?| instead of just \verb|.+| to do the last group?
\section{Portability Notes}
As mentioned in the Extra Credit you have to escape the parenthesis in
groups when you do this in Vim. Other regex engines use a slightly
different API and way of doing search/replace.