ref: a351bcdccdf5a4273bc8dc3360a48fbb8b8aa9ea
dir: /ch9.ms.old/
.so tmacs
.BC 9 "More tools
.BS 2 "Regular expressions
.LP
We have used
.CW sed
to replace one string with another. But, what happens here?
.P1
; echo foo.xcc | sed 's/.cc/.c/g'
foo..c
; echo focca.x | sed 's/.cc/.c/g'
f.ca.x
.P2
.LP
We need to learn more.
.PP
In addresses of the form
.CW /text/
and in commands like
.CW s/text/other/ ,
the string
.CW text
is not a string for
.CW sed .
This happens to many other programs that search for things.
For example, we have used
.CW grep
to print only lines containing a string. Well, the
.I string
given to grep, like in
.P1
; grep string file1 file2 ...
.P2
.LP
is
.I not
a string. It is a
.B "regular expression" .
A regular expression is a little language. It is very useful to master it, because
many commands employ regular expressions to let you do complex things
in an easy way.
.PP
The text in a regular expression represents many different strings. You have
already seen something similar. The
.CW *.c
in the shell, used for globbing, is very similar to a regular expression. Although
it has a slightly different meaning. But you know that in the shell,
.CW *.c
.B matches
with many different strings. In this case, those that are file names in the current
directory that happen to terminate with the characters “\f(CW.c\fP”. That is what
regular expressions, or
.I regexps ,
are for. They are used to select or match text, expressing the kind of text
to be selected in a simple way. They are a language on their own.
A regular expression, as known by
.CW sed ,
.CW grep ,
and many others,
is best defined recursively, as follows.
.IP •
Any single character
.I matches
the string consisting of that character. For example,
.CW a
matches
.CW a ,
but not
.CW b .
.IP •
A single dot, “\f(CW.\fP”, matches
.I any
single character. For example, “\f(CW.\fP” matches
.CW a
and
.CW b ,
but not
.CW ab .
.IP •
A set of characters, specified by writing a string within brackets, like
.CW [abc123] ,
matches
.I any
character in the string. This example would match
.CW a ,
.CW b ,
or
.CW 3 ,
but not
.CW x .
A set of characters, but starting with
.CW ^ ,
matches any character
.I not
in the set. For example,
.CW [^abc123]
matches
.CW x ,
but not
.CW 1 ,
which is in the string that follows the
.CW ^ .
A range may be used, like in
.CW [a-z0-9] ,
which matches any single character that is a letter or a digit.
.IP •
A single
.CW ^ ,
matches the start of the text. And a single
.CW $ ,
matches the end of the text. Depending on the program using the
regexp, the text may be a line or a file. For example, when using
.CW grep ,
.CW a
matches the character
.CW a
at
.I any
place. However,
.CW ^a
matches
.CW a
only when it is the first character in a line, and
.CW ^a$
also requires it to be the last character in the line.
.IP •
Two regular expressions concatenated match
any text matching the first regexp followed by any text
matching the second. This is more hard to say
than it is to understand. The expression
.CW abc
matches
.CW abc
because
.CW a
matches
.CW a ,
.CW b
matches
.CW b ,
and so on. The expression
.CW [a-z]x
matches any two characters where the first one matches
.CW [a-z] ,
and the second one is an
.CW x .
.IP •
Adding a
.CW *
after a regular expression, matches zero or any number of
strings that match the expression. For example,
.CW x*
matches the empty string, and also
.CW x ,
.CW xx ,
.CW xxx ,
etc. Beware,
.CW ab*
matches
.CW a ,
.CW ab ,
.CW abb ,
etc. But it does
.I not
match
.CW abab .
The
.CW *
applies to the preceding regexp, with is just
.CW b
in this case.
.IP •
Adding a
.CW +
after a regular expression, matches one or more
strings that match the previous regexp. It is like
.CW * ,
but there has to be at least one match. For example,
.CW x+
does not match the empty string, but it matches every other thing
matched by
.CW x* .
.IP •
Adding a
.CW ?
after a regular expression, matches either the empty string or
one string matching the expression. For example,
.CW x?
matches
.CW x
and the empty string. This is used to make parts optional.
.IP •
Different expressions may be surounded by parenthesis, to alter
grouping. For example,
.CW (ab)+
matches
.CW ab ,
.CW abab ,
etc.
.IP •
Two expressions separated by
.CW |
match anything matched either by the first, or the second regexp. For example,
.CW ab|xy
matches
.CW ab ,
and
.CW xy .
.IP •
A backslash removes the special meaning for any character used for
syntax. This is called a
.I escape
character.
For example,
.CW (
is not a well-formed regular expression, but
.CW \e(
is, and matches the string
.CW ( .
To use a backslash as a plain character, and not as a escape, use the
backslash to escape itself, like in
.CW \e\e .
.PP
That was a long list, but it is easy to learn regular expressions just by using
them. First, let's fix the ones we used in the last section. This is what happen to us.
.P1
; echo foo.xcc | sed 's/.cc/.c/g'
foo..c
; echo focca.x | sed 's/.cc/.c/g'
f.ca.x
.P2
.LP
But we wanted to replace
.CW .cc ,
and not
.I any
character and a
.CW cc .
Now we know that the first argument to the
.CW sed
command
.CW s ,
is a regular expression.
We can try to fix our problem.
.P1
; echo foo.xcc | sed 's/\e.cc/.c/g'
foo.xcc
; echo focca.x | sed 's/\e.cc/.c/g'
focca.x
.P2
.LP
It seems to work. The backslash removes the special meaning for the dot,
and makes it match just one dot. But this may still happen.
.P1
; echo foo.cc.x | sed 's/\e.cc/.c/g'
foo.c.x
.P2
.LP
And we wanted to replace only the extension for file names ending in
.CW .cc .
We can modify our expression to match
.CW .cc
only when immediately before the end of the line (which is the string being
matched here).
.P1
; echo foo.cc.x | sed 's/\e.cc$/.c/g'
foo.cc.x
; echo foo.x.cc | sed 's/\e.cc/.c/g'
foo.x.c
.P2
.LP
Sometimes, it is useful to be able to refer to text that matched part
of a regular expression. Suppose you want to replace the variable name
.CW text
with
.CW word
in a program.
You might try with
.CW s/text/word/g ,
but it would change other identifiers, which is not what you want.
.P1
; cat f.c
void
printtext(char* text)
{
print("[%s]", text);
}
; sed 's/text/word/g' f.c
void
printword(char* word)
{
print("[%s]", word);
}
.P2
.LP
The change is only to be done if
.CW word
is not surounded by characters that may be part of an identifier in the
program. For simplicity, we will assume that these characters are just
.CW [a-z0-9_] .
We can do what follows.
.P1
; sed 's/([^a-z0-9_])text([^a-z0-9_])/\e1word\e2/g' f.c
void
printtext(char* word)
{
print("[%s]", word);
}
.P2
.LP
The regular expression
.CW [^a-z0-9_]text[^a-z0-9_]
means “any character that may not be part of an identifier”, then
.CW text ,
and then “any character that may not be part of an identifier”.
Because the substitution affects
.I all
the regular expression, we need to substitute the matched string with
another one that has
.CW word
instead of
.CW text ,
but keeping the characters matching
.CW [^a-z0-9_]
before and after the string
.CW text .
This can be done by surounding in parentheses both
.CW [^a-z0-9_] .
Later, in the destination string, we may use
.CW \e1
to refer to the text matching the first regexp within parenthesis, and
.CW \e2
to refer to the second.
.PP
Because
.CW printtext
is not matched by
.CW [^a-z0-9_]text[^a-z0-9_] ,
it was untouched. However, “\f(CW␣text)\fP” was matched. In the destination string,
.CW \e1
was a white space,
because that is what matched the first parenthesized part. And
.CW \e2
was a right parenthesis, because that is what matched the second one.
As a result, we left those characters untouched, and used them as
.I context
to determine when to do the substitution.
.PP
Regular expressions permit to clean up source files in an easy way.
In may cases, it makes no sense to keep white space at the end of lines.
This removes them.
.P1
; sed 's/[ \t]*//'
.P2
.LP
We saw that a script
.CW t+
can be used to indent text in Acme.
Here it is.
.P1
; cat /bin/t+
#!/bin/rc
sed 's/^/\t/'
;
.P2
.LP
This other script removes one level of indentation.
.P1
; cat /bin/t-
#!/bin/rc
sed 's/^\t//'
;
.P2
.LP
There are many other useful uses of regular expressions, as you will be
able to see from here to the end of this book. In many cases, your C
programs can be made more flexible by accepting regular expressions
for certain parameters instead of mere strings. For example, an editor
might accept a regular expression that determines if the text is to be
shown using a
.CW "constant width font"
or a
.I "proportional width font" .
For file names matching, say
.CW .*\e.[ch] ,
it could use a constant width font, to show C source code like you
might be
acustomed to see it.
.PP
It turns out that it is
.I trivial
to use regular expressions in a C program, by using the
.CW regexp
library. The expression is
.I compiled
into a description more amenable to the machine, and the resulting
data structure (called a
.CW Reprog )
can be used for matching strings against the expression. This program
accepts a regular expression as a parameter, and then reads one line
at a time. For each such line, it reports if the string read matches the
regular expression or not.
.so src/match.c.ms
.LP
The call to
.CW regcomp
.I compiles
the regular expression into
.CW prog .
Later,
.CW regexec
.I executes
the compiled regular expression to determine if it matches the string
just read in
.CW buf .
The parameter
.CW sub
points to an array of structures that keeps information about the match.
The whole string matching starts at the character pointed to by
.CW sub[0].sp
and terminates right before the one pointed to by
.CW sub[0].ep .
Other entries in the array report which substring matched the
first parenthesized expression in the regexp,
.CW sub[1] ,
which one matched the second one,
.CW sub[2] ,
etc. They are similar to
.CW \e1 ,
.CW \e2 ,
etc.
This is an example session with the program.
.P1
; 8.out '*.c' \fRThe * needs something on the left!\fP
regerror: missing operand for *
; 8.match '\e.[123]'
!!x123
no match
!!.123
matched: '.1'
!!x.z
no match
!!x.3
matched: '.3'
.P2
.BS 2 "Sorting and searching
.LP
One of the most useful task achieved with a few shell commands is inspecting
the system to find out things. In what follows we are going to learn how to do this,
using several assorted examples.
.PP
Running out of disk space? It is not likely, given the big disks we have today.
But anyway, which ones are the biggest files you have created at your home
directory?
.PP
The command
.CW du
reports disk usage, measured in disk blocks. A disk block is usually 8 or 16 Kbytes,
depending on your file system. Although
.CW "du -a"
reports the size in blocks for each file, it is a burden to scan by yourself through the
whole list of files to search for the biggest one. The command
.CW sort
is used to sort lines of text, according to some criteria. We can ask
.CW sort
to sort the output of
.CW du
in numerically decreasing order (biggest numbers first) and then use
.CW sed
to print just the first few lines. Those ones correspond to the biggest files, which
we are interested in.
.P1
; du -a bin | sort -nr | sed 15q
4211 bin
3085 bin/arm
864 bin/arm/enc
834 bin/386
333 bin/arm/madplay
320 bin/arm/madmix
319 bin/arm/deco
316 bin/386/minimad
316 bin/arm/minimad
280 bin/arm/mp3
266 bin/386/minisync
258 bin/rc
212 bin/arm/calc
181 bin/arm/mpg123
146 bin/386/r2bib
;
.P2
.LP
This includes directories as well, but point us quickly to files like
.CW bin/arm/enc
that seem to occupy 864 disk blocks!
.PP
But in any case, if the disk is filling up,
it is a good idea to locate the users that created files (or added data to them),
to alert them. The flag
.CW -m
for
.CW ls
lists the user name that last modified the file. We may collect user names for
all the files in the disk, and then notify them. We are going to play with commands
until we complete our task, using
.CW sed
to print just a few lines until we know how to process all the information.
The first step is to use the output
of
.CW du
as the initial data, the list of files. If we remove everything up to the file names,
we obtain a list of files to work with.
.P1
; du -a bin | sed 's/.* //' | sed 3q
bin/386/minimad
bin/386/minisync
bin/386/r2bib
.P2
.LP
Now we want to list the user who modified each file. We can change our data
to produce the commands that do that, and send them to a shell.
.P1
; du -a bin | sed 's/.* //' | sed 's/^/ls -m /' | sed 3q
ls -m bin/386/minimad
ls -m bin/386/minisync
ls -m bin/386/r2bib
;
; du -a bin | sed 's/.* //' | sed 's/^/ls -m /' | sed 3q | rc
[nemo] bin/386/minimad
[none] bin/386/minisync
[nemo] bin/386/r2bib
;
.P2
.LP
We still have to work a little bit more. And our command line is growing. Being
able to edit the text at any place in a Rio window does help, but it can be
convenient to define a
.B "shell function"
that encapsulates what we have done so far. A shell function is like a function
in any other language. The difference is that a shell function receives arguments
as any other command, in the command line. Besides, a shell function has
command lines in its body, which is not a surprise. Defining a function
for what we have done so far can save some typing in the near future.
Furthermore, the command we have just built, to list all the files within a given
directory, is useful by itself.
.P1
; fn lr {
;; du -a $1 | sed 's/.* //' | sed 's/^/ls -m /' | rc
;; }
;
.P2
.LP
This defined a function, named
.CW lr ,
that executes exactly the command line we developed. Rc stores the function
definition using an
environment variable. Thus, most things said for environment variables apply for
functions as well (e.g., think about
.CW "rfork e" ).
.P1
; cat /env/'fn#lr'
fn lr {du -a $1|sed 's/.* //'|sed 's/^/ls -m /'|rc}
;
.P2
.LP
In the function, we removed the
.CW "sed 3q"
because it is not reasonable for a function listing all files recursively to
stop after listing three of them. If we want to play, we can always add a
final
.CW sed
in a pipeline. Arguments given to the function are accessed like they would be
in a shell script. The difference is that the function is executed by the shell
where we invoque it, and not by a child shell. By the way, it is preferable to create
useful commands by creating ia shell, functions can not be edited as scripts, and
are not automatically shared among all shells like files are. Functions are handy to
make modular scripts. Let's try our new function.
.P1
; lr bin
[nemo] bin/386/minimad
[none] bin/386/minisync
[nemo] bin/386/r2bib
[nemo] bin/386/rc2bin
.I "...and many other lines of output..."
;
.P2
.LP
To obtain our list of users, we may remove everything but the user name.
.P1
; lr bin | sed 's/.([a-z0-9]+).*/\e1/' | sed 3q
nemo
none
nemo
;
.P2
.LP
And now, to get a list of users, we must drop dupplicates. The program
.CW uniq
knows how to do it, it reads lines and prints them, lines showing up more than once
in the input are printed once. This program
needs an input with sorted lines. Therefore, we do what we just did, and sort
the lines and remove dupplicate ones.
.P1
; lr bin | sed 's/.([a-z0-9]+).*/\e1/' | sort | uniq
esoriano
nemo
none
;
.P2
.LP
Note that we removed
.CW "sed 3q"
from the pipeline, because this command does what we wanted to do and
we want to process the whole file tree, and not just the first three ones.
.PP
To do something different,
we can revisit the first example in the last chapter, finding function definitions.
This script does just that, if we follow the style convention for declaring
functions that was shown at the beginning of this chapter. First, we try
to use
.CW grep
to print just the source line where the function
.CW cat
is defined in the file
.CW /sys/src/cmd/cat.c .
Our first try is this.
.P1
; grep cat /sys/src/cmd/cat.c
cat(int f, char *s)
argv0 = "cat";
cat(0, "<stdin>");
cat(f, argv[i]);
.P2
.LP
Which is not too helpful. All the lines contain the string
.CW cat ,
but we want only the lines where
.CW cat
is at the beggining of line, followed by an open parenthesis. Second
attempt.
.P1
; grep '^cat\e(' /sys/src/cmd/cat.c
cat(int f, char *s)
.P2
.LP
At least, this prints just the line of interest to us. However, it is useful
to get the file name and line number before the text in the line. That
output can be used to point an editor to that particular file and line
number. Because
.CW grep
prints the file name when more than one file is given, we could use
.CW /dev/null
as a second file where to search for the line. It would not be there,
but it would make
.CW grep
print the file name.
.P1
; grep '^cat\e(' /sys/src/cmd/cat.c /dev/null
/sys/src/cmd/cat.c:cat(int f, char *s)
.P2
.LP
Giving the option
.CW -n
to
.CW grep
makes it print the line number. Now we can really search for functions,
like we do next.
.P1
; grep -n '^cat\e(' /sys/src/cmd/*.c
/sys/src/cmd/cat.c:5: cat(int f, char *s)
.P2
.LP
And because this seems useful, we can package it as a shell script. It
accepts as arguments the names for functions to be located. The command
.CW grep
is used to search for such functions at all the source files in the current directory.
.P1
#!/bin/rc
rfork e
for (f in $*)
grep -n '^'$f'\e(' *.[cCh]
.P2
.LP
How can we use
.CW grep
to search for
.CW -n ?
If we try,
.CW grep
would get confussed, thinking that we are supplying an option. To avoid this,
the
.CW -e
option tells
.CW grep
that what follows is a regexp to search for.
.P1
; cat text
Hi there
How can we grep for -n?
Who knows!
; grep -n text
; grep -e -n text
how can we grep for -n?
.P2
.LP
This program has other useful options. For example, if may want to
locate lines in the file for a chapter of this book where we mention
figures. However, if the word
.CW figure
is in the middle of a sentence it would be all lowercase. When it is
starting a sentece, it would be capitalized. We must search both for
.CW Figure
and
.CW figure.
The flag
.CW -i
makes
.CW grep
become case-insensitive. All the text read is converted to lowercase
before matching the expression.
.P1
; grep -i figure ch1.ms
Each window shows a file or the output of commands. Figure
figure are understood by acme itself. For commands
shown in the figure would be
.I "...and other matching lines
.P2
.LP
A popular searching task is determining if a file containing a mail message
is spam or not. Today, it would not work, because spammers employ heavy
armoring, and even send their text encoded in multiple images sent as HTML
mail. However, it was popular to see if a mail message contained certain
expressions, if it did, it was considered spam.
Because there will be many expressions, we may keep them in a file.
The option
.CW -f
for
.CW grep
takes as an argument a file containing all the expressions to search for.
.P1
; cat patterns
Make money fast!
Earn 10+ millions
(Take|use) viagra for a (better|best) life.
; if (grep -i -f patterns $mailfile ) echo $mailfile is spam
.P2
.LP
A different kind of search is looking for differences. Consider that you have
mounted at
.CW /n/sources/plan9
the Plan 9 distribution file server, known as
.I sources .
You Plan 9 is likely to be installed by pulling changes from sources and
updating your file tree. Nevertheless, you might want to be sure that a
particular part of your file tree is exactly the same as it is in sources.
.PP
There are
several tools that can be used to compare files. We saw
.CW cmp ,
but it compares only two files. Besides,
.CW cmp
does not give much information, because it is meant to compare files that
are binary and not textual, and the program reports just which one is the first
byte that makes the files different. However, there is another tool,
.CW diff ,
that is just what we need. For example, we can compare
.PP
Show diff, to search for differences.
See the script bdiff
.BS 2 "AWK
.PP
A simple calculator.
Pickup some examples from your bin
.BS 2 "More complex things
.PP
Show the scripts to replace strings.
Show the script to automatically add users from a list
of people.
.PP
tal vez seria mejor una seccion en procesar ficheros con campos.
y ver alli sort uniq y demas. O a lo mejor no. falta por ver awk, esa
seccion es una oportunidad para usar alli mismo sort uniq y demas.
tail -f
Show eval. Use walk as an example.
Show doctype also.
must talk about other file systems,
plumber,
cdfs,
tarfs,
upasfs,
replica
the dump
.PP