An expression is a string of characters. Those characters having an interpretation above and beyond their literal meaning are called metacharacters. A quote symbol, for example, may denote speech by a person, ditto, or a meta-meaning [98] for the symbols that follow. Regular Expressions are sets of characters and/or metacharacters that match (or specify) patterns.
A Regular Expression contains one or more of the following:
A character set. These are the characters retaining their literal meaning. The simplest type of Regular Expression consists only of a character set, with no metacharacters.
An anchor. These designate (anchor) the position in the line of text that the RE is to match. For example, ^, and $ are anchors.
Modifiers. These expand or narrow (modify) the range of text the RE is to match. Modifiers include the asterisk, brackets, and the backslash.
The main uses for Regular Expressions (REs) are text searches and string manipulation. An RE matches a single character or a set of characters -- a string or a part of a string.
The asterisk -- * -- matches any number of repeats of the character string or RE preceding it, including zero instances.
“1133*” matches 11 +
one or more 3's:
113, 1133,
1133333, and so forth.
The dot -- . -- matches any one character, except a newline. [99]
“13.” matches 13 + at
least one of any character (including a
space): 1133,
11333, but not
13 (additional character
missing).
See Example 16.18, “Crossword puzzle solver” for a demonstration of dot single-character matching.
The caret -- ^ -- matches the beginning of a line, but sometimes, depending on context, negates the meaning of a set of characters in an RE.
The dollar sign -- $ -- at the end of an RE matches the end of a line.
“XXX$” matches XXX at the end of a line.
“^$” matches blank lines.
Brackets -- [...] -- enclose a set of characters to match in a single RE.
“[xyz]” matches any one of the characters
x, y,
or z.
“[c-n]” matches any one of the
characters in the range c
to n.
“[B-Pk-y]” matches any one of the
characters in the ranges B
to P
and k to
y.
“[a-z0-9]” matches any single lowercase letter or any digit.
“[^b-d]” matches any character
except those in
the range b to
d. This is an instance of
^ negating or inverting the meaning
of the following RE (taking on a role similar to
! in a different context).
Combined sequences of bracketed characters match
common word patterns. “[Yy][Ee][Ss]” matches
yes, Yes,
YES, yEs,
and so forth.
“[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]”
matches any Social Security number.
The backslash -- \ -- escapes a special character, which means that character gets interpreted literally (and is therefore no longer special).
A “\$” reverts back to its literal meaning of “$”, rather than its RE meaning of end-of-line. Likewise a “\\” has the literal meaning of “\”.
Escaped “angle brackets” -- \<...\> -- mark word boundaries.
The angle brackets must be escaped, since otherwise they have only their literal character meaning.
“\<the\>” matches the word “the,” but not the words “them,” “there,” “other,” etc.
bash$cat textfileThis is line 1, of which there is only one instance. This is the only instance of line 2. This is line 3, another line. This is line 4.bash$grep 'the' textfileThis is line 1, of which there is only one instance. This is the only instance of line 2. This is line 3, another line.bash$grep '\<the\>' textfileThis is the only instance of line 2.
Extended REs. Additional metacharacters added to the basic set. Used in egrep, awk, and Perl.
The question mark -- ? -- matches zero or one of the previous RE. It is generally used for matching single characters.
The plus -- + -- matches one or more of the previous RE. It serves a role similar to the *, but does not match zero occurrences.
# GNU versions of sed and awk can use "+", # but it needs to be escaped. echo a111b | sed -ne '/a1\+b/p' echo a111b | grep 'a1\+b' echo a111b | gawk '/a1+b/' # All of above are equivalent. # Thanks, S.C.
Escaped “curly brackets” -- \{ \} -- indicate the number of occurrences of a preceding RE to match.
It is necessary to escape the curly brackets since they have only their literal character meaning otherwise. This usage is technically not part of the basic RE set.
“[0-9]\{5\}” matches exactly five digits (characters in the range of 0 to 9).
Curly brackets are not available as an RE in the
“classic” (non-POSIX compliant) version
of awk.
However, the GNU extended version
of awk, gawk,
has the --re-interval option that permits
them (without being escaped).
bash$echo 2222 | gawk --re-interval '/2{3}/'2222
Perl and some egrep versions do not require escaping the curly brackets.
Parentheses -- ( ) -- enclose a group of REs. They are useful with the following “|” operator and in substring extraction using expr.
The -- | -- “or” RE operator matches any of a set of alternate characters.
bash$egrep 're(a|e)d' misc.txtPeople who read seem to be better informed than those who do not. The clarinet produces sound by the vibration of its reed.
Some versions of sed, ed, and ex support escaped versions of the extended Regular Expressions described above, as do the GNU utilities.
POSIX Character Classes. [:class:]
This is an alternate method of specifying a range of characters to match.
[:alnum:] matches alphabetic or
numeric characters. This is equivalent to
A-Za-z0-9.
[:alpha:] matches alphabetic
characters. This is equivalent to
A-Za-z.
[:blank:] matches a space or a
tab.
[:cntrl:] matches control
characters.
[:digit:] matches (decimal)
digits. This is equivalent to
0-9.
[:graph:] (graphic printable
characters). Matches characters in the range of ASCII 33 - 126. This is
the same as [:print:], below,
but excluding the space character.
[:lower:] matches lowercase
alphabetic characters. This is equivalent to
a-z.
[:print:] (printable
characters). Matches characters in the range of ASCII 32 -
126. This is the same as [:graph:],
above, but adding the space character.
[:space:]
matches whitespace characters (space and horizontal
tab).
[:upper:] matches uppercase
alphabetic characters. This is equivalent to
A-Z.
[:xdigit:] matches hexadecimal
digits. This is equivalent to
0-9A-Fa-f.
POSIX character classes generally require quoting or double brackets ([[ ]]).
bash$grep [[:digit:]] test.fileabc=723
# ... if [[ $arow =~ [[:digit:]] ]] # Numerical input? then # POSIX char class if [[ $acol =~ [[:alpha:]] ]] # Number followed by a letter? Illegal! # ... # From ktour.sh example script.
These character classes may even be used with globbing, to a limited extent.
bash$ls -l ?[[:digit:]][[:digit:]]?-rw-rw-r-- 1 bozo bozo 0 Aug 21 14:47 a33b
POSIX character classes are used in Example 16.21, “toupper: Transforms a file to all uppercase.” and Example 16.22, “lowercase: Changes all filenames in working directory to lowercase.”.
Sed, awk, and Perl, used as filters in scripts, take REs as arguments when "sifting" or transforming files or I/O streams. See Example A.12, “behead: Removing mail and news message headers” and Example A.16, “tree: Displaying a directory tree” for illustrations of this.
The standard reference on this complex topic is Friedl's Mastering Regular Expressions. Sed & Awk, by Dougherty and Robbins, also gives a very lucid treatment of REs. See the Bibliography for more information on these books.
[98] A meta-meaning is the meaning of a term or expression on a higher level of abstraction. For example, the literal meaning of regular expression is an ordinary expression that conforms to accepted usage. The meta-meaning is drastically different, as discussed at length in this chapter.
[99] Since sed, awk, and grep process single lines, there will usually not be a newline to match. In those cases where there is a newline in a multiple line expression, the dot will match the newline.
#!/bin/bash
sed -e 'N;s/.*/[&]/' << EOF # Here Document
line1
line2
EOF
# OUTPUT:
# [line1
# line2]
echo
awk '{ $0=$1 "\n" $2; if (/line.1/) {print}}' << EOF
line 1
line 2
EOF
# OUTPUT:
# line
# 1
# Thanks, S.C.
exit 0