An expression is a string of characters. Those characters
that have 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 for the symbols
that follow. Regular Expressions are sets of characters and/or
metacharacters that UNIX endows with special features.
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 substring or an entire string).
The asterisk -- * -- matches any number of
repeats of the character string or RE preceding it,
including zero.
"1133*" matches 11 +
one or more 3's + possibly other characters:
113, 1133,
111312, and so forth.
The dot -- . -- matches
any one character, except a newline.
"13." matches 13 + at
least one of any character (including a
space): 1133,
11333, but not
13 (additional character
missing).
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.
"^$" matches blank lines.
 | The ^ and $ are known as
anchors, since they indicate or anchor a
position within an RE. |
Brackets -- [...] -- enclose a set of characters
to match in a single RE.
"[xyz]" matches the characters
x, y,
or z.
"[c-n]" matches any of the
characters in the range c
to n.
"[B-Pk-y]" matches any of the
characters in the ranges B
to P
and k to
y.
"[a-z0-9]" matches any lowercase letter or any
digit.
"[^b-d]" matches all characters
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.
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 textfile
This 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' textfile
This 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\>' textfile
This is the only instance of line 2.
|
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.file
abc=723
|
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
|
To see POSIX character classes used in scripts, refer to
Example 12-14 and Example 12-15. |
"Sed & Awk", by Dougherty and Robbins gives a very complete
and lucid treatment of REs (see the Bibliography).
