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Next: 6. Editing Text Files Up: rute Previous: 4. Basic Commands Contents
Subsections
- 5.1 Overview
- 5.2 The
fgrepCommand - 5.3 Regular Expression
\{\}Notation - 5.4
+?\<\>()|Notation - 5.5 Regular Expression Subexpressions
5. Regular Expressions
A regular expression is a sequence of characters that forms a template used to search for strings [Words, phrases, or just about any sequence of characters. ] within text. In other words, it is a search pattern. To get an idea of when you would need to do this, consider the example of having a list of names and telephone numbers. If you want to find a telephone number that contains a 3 in the second place and ends with an 8, regular expressions provide a way of doing that kind of search. Or consider the case where you would like to send an email to fifty people, replacing the word after the ``Dear'' with their own name to make the letter more personal. Regular expressions allow for this type of searching and replacing.
5.1 Overview
Many utilities use the regular expression to give them greater power when manipulating text.
The
grep command is an example. Previously you used the
grep
command to locate only simple letter sequences in text. Now we will use it to
search for regular expressions.
In the previous chapter you learned that the
? character can be used to signify
that any character can take its place. This is said to be a wildcard
and works with file names. With regular expressions, the wildcard to use is the
. character. So, you can use the command
grep .3....8 <filename>
to find the seven-character telephone number that you are looking for in the
above example.
Regular expressions are used for line-by-line searches. For instance, if the
seven characters were spread over two lines (i.e., they had a line break in the
middle), then
grep wouldn't find them. In general, a program that uses
regular expressions will consider searches one line at a time.
Here are some regular expression examples that will teach you the regular expression basics. We
use the
grep command to show the use of regular expressions (remember
that the
-w option matches whole words only). Here the expression itself
is enclosed in
' quotes for reasons that are explained later.
grep -w 't[a-i]e'- Matches the words
tee,the, andtie. The brackets have a special significance. They mean to match one character that can be anything fromatoi. grep -w 't[i-z]e'- Matches the words
tieandtoe. grep -w 'cr[a-m]*t'- Matches the words
craft,credit, andcricket. The*means to match any number of the previous character, which in this case is any character fromathroughm. grep -w 'kr.*n'- Matches the words
kremlinandkrypton, because the.matches any character and the*means to match the dot any number of times. egrep -w '(th|sh).*rt'- Matches the words
shirt,short, andthwart. The|means to match either thethor thesh.egrepis just likegrepbut supports extended regular expressions that allow for the|feature. [ The|character often denotes a logicalOR, meaning that either the thing on the left or the right of the|is applicable. This is true of many programming languages. ] Note how the square brackets mean one-of-several-characters and the round brackets with|'s mean one-of-several-words. grep -w 'thr[aeiou]*t'- Matches the words
threatandthroat. As you can see, a list of possible characters can be placed inside the square brackets. grep -w 'thr[^a-f]*t'- Matches the words
throughputandthrust. The ^ after the first bracket means to match any character except the characters listed. For example, the wordthriftis not matched because it contains anf.
The above regular expressions all match whole words (because of the
-w
option). If the
-w option was not present, they might match parts of
words, resulting in a far greater number of matches. Also note that
although the
* means to match any number of characters, it also will match
no characters as well; for example:
t[a-i]*e could actually
match the letter sequence
te, that is, a
t and an
e with
zero characters between them.
Usually, you will use regular expressions to search for whole lines that
match, and sometimes you would like to match a line that begins or ends with
a certain string. The
^ character specifies the beginning
of a line, and the
$ character the end of the line. For example,
^The matches all lines that start with a
The, and
hack$
matches all lines that end with
hack, and
'^ *The.*hack
*$' matches all lines that begin with
The and end with
hack,
even if there is whitespace at the beginning or end of the line.
Because regular expressions use certain characters in a special way (these are
.
\
[
]
*
+
?), these characters cannot be used to match characters.
This restriction severely limits you from trying to match, say, file names, which
often use the
. character. To match a
. you can use the sequence
\. which forces interpretation as an actual
.
and not as a wildcard. Hence, the regular expression
myfile.txt might
match the letter sequence
myfileqtxt or
myfile.txt, but the
regular expression
myfile\.txt will match only
myfile.txt.
You can specify most special characters by adding a
\ character
before them, for example, use
\[ for an actual
[,
a
\$ for an actual
$, a
\\
for and actual
\,
\+ for an
actual
+, and
\? for an actual
?.
(
? and
+ are explained below.)
5.2 The
fgrep Command
fgrep is an alternative to
grep. The difference is that while
grep (the more commonly used command) matches regular expressions,
fgrep matches literal strings. In other words you can use
fgrep
when you would like to search for an ordinary string that is not a regular
expression, instead of preceding special characters with
\.
5.3 Regular Expression
\{
\} Notation
x* matches zero to infinite instances of a character
x.
You can specify other ranges of numbers of characters to be matched with, for
example,
x\{3,5\}, which will match
at least three but not more than five
x's, that is
xxx,
xxxx,
or
xxxxx.
x\{4\} can then be used to match 4
x's exactly: no more and no less.
x\{7,\}
will match seven or more
x's--the upper limit is omitted to mean
that there is no maximum number of
x's.
As in all the examples above, the
x can be a range of characters (like
[a-k])
just as well as a single charcter.
grep -w 'th[a-t]\{2,3\}t'- Matches
the words
theft,thirst,threat,thrift, andthroat. grep -w 'th[a-t]\{4,5\}t'- Matches
the words
theorist,thicket, andthinnest.
5.4 Extended Regular Expression
+
?
\<
\>
(
)
|
Notation with
egrep
An enhanced version of regular expressions allows for a few more
useful features. Where these conflict with existing notation, they are only
available through the
egrep command.
- +
- is analogous to
\{1,\}. It does the same as*but matches one or more characters instead of zero or more characters. ?- is analogous to \{1\}. It matches zero or one character.
\< \>- can surround a string to match only whole words.
()- can surround several strings, separated by
|. This notation will match any of these strings. (egreponly.) \(\)- can surround several strings, separated by
\|. This notation will match any of these strings. (greponly.)
The following examples should make the last two notations clearer.
grep 'trot'- Matches the words
electrotherapist,betroth, and so on, but grep '\<trot\>'- matches only the word
trot. egrep -w '(this|that|c[aeiou]*t)'- Matches the words
this,that,cot,coat,cat, andcut.
5.5 Regular Expression Subexpressions
Subexpressions are covered in Chapter 8.
Next: 6. Editing Text Files Up: rute Previous: 4. Basic Commands Contents