Patching existing commands

In the general case (possibly sticking something in the middle of an existing command) this is difficult. However, the common requirement, to add some code at the beginning or the end of an existing command, is conceptually quite easy. Suppose we want to define a version of a command that does some small extension of its original definition: we would naturally write: 

\renewcommand{\splat}{\mumble\splat}

However, this would not work: a call to \splat would execute \mumble, and then call the redefined \splat again; this is an “unterminated recursion”, that will quickly exhaust TeX's memory.

Fortunately, the TeX primitive \let command comes to our rescue; it allows us to take a “snapshot” of the current state of a command, which we can then use in the redefinition of the command. So: 

\let\OldSmooth\smooth
\renewcommand{\smooth}{\mumble\OldSmooth}

effects the required patch, safely. Adding things at the end of a command works similarly.

If \smooth takes arguments, one must pass them on: 

\let\OldSmooth\smooth
\renewcommand{\smooth}[2]{\mumble\OldSmooth{#1}{#2}}

The situation is more difficult still if \smooth takes an optional argument; the structure of the command is so complex that the simple \let command does not retain the necessary detail. In this case, we need the letltxmacro package which knows about all sorts of LaTeX commands and how to replicate them. Suppose we have a command defined as: 

\newcommand{\rough}[1][\default]{...}

with an optional argument (substituted with \default if it's not present) as well as an ordinary one. In this case we copy it using 

\LetLtxMacro{\NewRough}{\rough}

and then repeat the technique we had above, with one extension: 

\renewcommand{\rough}[1][\newdef]%
             {\mumble\OldRough[{#1}]{#2}}

We see here that (for tedious argument-matching reasons) it is necessary to provide braces arround the optional argument that is being passed on.

The general case may be achieved in two ways. First, one can use the LaTeX command \CheckCommand; this compares an existing command with the definition you give it, and issues a warning if two don't match. Use is therefore: 

\CheckCommand{\complex}{⟨original definition⟩}
 
\renewcommand{\complex}{⟨new definition⟩}

This technique is obviously somewhat laborious, but if the original command comes from a source that's liable to change under the control of someone else, it does at least warn you that your patch is in danger of going wrong.

Otherwise, LaTeX users may use one of the patchcmd, ted or etoolbox packages.

The patchcmd package addresses a slightly simpler task, by restricting the set of commands that you may patch; you mayn't patch any command that has an optional argument, though it does deal with the case of commands defined with \DeclareRobustCommand. The package defines a \patchcommand command, that takes three arguments: the command to patch, stuff to stick at the front of its definition, and stuff to stick on the end of its definition. So, after 

\def\b{b}
\patchcmd\b{a}{c}

we will have a new version of \b defined as abc.

The ted package is a “token list editor”, and provides a command \substitute which will patch the contents of a macro, putting the result in a token-list, or (in the form \Substitute*) using the result to (re)define a macro. The following example defines a simple macro, and then changes its definition: 

\newcommand{\myfont}%
    {\Large\sffamily\selectfont}
\Substitute*[\renewcommand{\myfont}]{\myfont}%
    {\Large\sffamily}{\huge\itshape}

The macro's definition is now: 

\huge\itshape\selectfont

The package also offers a command \ShowTokens, which shows the content of its argument, one token to a line, with details of the token's category code, etc. This is recommended as a debugging tool.

The etoolbox package (which provides user access to $\epsilon$-TeX's programming facilities) provides a command \patchcmd which is very similar to \Substitute, except that it only replaces a single instance of the token(s) in its search pattern. Since not all commands may be patched in this way, \patchcmd takes extra arguments for success and failure cases. The package also provides commands that prepend (\pretocmd) or append (\apptocmd) to the definition of a command. Not all commands may be patched in this way; the package provides a command \ifpatchable which checks the prerequisites, and checks that the target command's body does indeed include the patch string you propose to use. (The command \ifpatchable* omits the test on the patch string.)

The regexpatch package deals with cases that are inaccessible with etoolbox; it uses the regular expression (pattern-matching) package l3regex from the LaTeX3 distribution to find the code you need to patch. The package also “knows about” robust commands and about biblatex.

Finally, we'll briefly consider a package that is (just about) usable, but not really recommendable. Patch gives you an ingenious (and difficult to understand) mechanism, and comes as an old-style LaTeX documented macro file, which can no longer be processed to produce formatted documentation, etc.. Fortunately, the file (patch.doc) may be input directly, and “documentation” may found by reading the source of the package. Roughly speaking, one gives the command a set of instructions analogous to sed substitutions, and it regenerates the command thus amended. Unless you can't do your job any other way, patch is best avoided.

Source: Patching existing commands