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A regular expression, or regexp, is a way of describing a
set of strings.
Because regular expressions are such a fundamental part of awk
programming, their format and use deserve a separate chapter.
A regular expression enclosed in slashes (`/')
is an awk pattern that matches every input record whose text
belongs to that set.
The simplest regular expression is a sequence of letters, numbers, or
both. Such a regexp matches any string that contains that sequence.
Thus, the regexp `foo' matches any string containing `foo'.
Therefore, the pattern /foo/ matches any input record containing
the three characters `foo' anywhere in the record. Other
kinds of regexps let you specify more complicated classes of strings.
Initially, the examples in this chapter are simple. As we explain more about how regular expressions work, we will present more complicated instances.
3.1 How to Use Regular Expressions 3.2 Escape Sequences How to write non-printing characters. 3.3 Regular Expression Operators 3.4 Using Character Lists What can go between `[...]'. 3.5 gawk-Specific Regexp OperatorsOperators specific to GNU software. 3.6 Case Sensitivity in Matching How to do case-insensitive matching. 3.7 How Much Text Matches? How much text matches. 3.8 Using Dynamic Regexps
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A regular expression can be used as a pattern by enclosing it in slashes. Then the regular expression is tested against the entire text of each record. (Normally, it only needs to match some part of the text in order to succeed.) For example, the following prints the second field of each record that contains the string `foo' anywhere in it:
$ awk '/foo/ { print $2 }' BBS-list
-| 555-1234
-| 555-6699
-| 555-6480
-| 555-2127
|
Regular expressions can also be used in matching expressions. These
expressions allow you to specify the string to match against; it need
not be the entire current input record. The two operators `~'
and `!~' perform regular expression comparisons. Expressions
using these operators can be used as patterns, or in if,
while, for, and do statements.
(See section Control Statements in Actions.)
For example:
exp ~ /regexp/ |
is true if the expression exp (taken as a string) matches regexp. The following example matches, or selects, all input records with the uppercase letter `J' somewhere in the first field:
$ awk '$1 ~ /J/' inventory-shipped -| Jan 13 25 15 115 -| Jun 31 42 75 492 -| Jul 24 34 67 436 -| Jan 21 36 64 620 |
So does this:
awk '{ if ($1 ~ /J/) print }' inventory-shipped
|
This next example is true if the expression exp (taken as a character string) does not match regexp:
exp !~ /regexp/ |
The following example matches, or selects, all input records whose first field does not contain the uppercase letter `J':
$ awk '$1 !~ /J/' inventory-shipped -| Feb 15 32 24 226 -| Mar 15 24 34 228 -| Apr 31 52 63 420 -| May 16 34 29 208 ... |
When a regexp is enclosed in slashes, such as /foo/, we call it
a regexp constant, much like 5.27 is a numeric constant and
"foo" is a string constant.
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Some characters cannot be included literally in string constants
("foo") or regexp constants (/foo/).
Instead, they should be represented with escape sequences,
which are character sequences beginning with a backslash (`\').
One use of an escape sequence is to include a double quote character in
a string constant. Because a plain double quote ends the string, you
must use `\"' to represent an actual double quote character as a
part of the string. For example:
$ awk 'BEGIN { print "He said \"hi!\" to her." }'
-| He said "hi!" to her.
|
The backslash character itself is another character that cannot be
included normally; you must write `\\' to put one backslash in the
string or regexp. Thus, the string whose contents are the two characters
`"' and `\' must be written "\"\\".
Another use of backslash is to represent unprintable characters such as tab or newline. While there is nothing to stop you from entering most unprintable characters directly in a string constant or regexp constant, they may look ugly.
The following table lists
all the escape sequences used in awk and
what they represent. Unless noted otherwise, all these escape
sequences apply to both string constants and regexp constants:
\\
\a
\b
\f
\n
\r
\t
\v
\nnn
\xhh...
awk.)
\/
awk to keep processing the rest of the regexp.
\"
awk to keep processing the rest of the string.
In gawk, a number of additional two-character sequences that begin
with a backslash have special meaning in regexps.
See section gawk-Specific Regexp Operators.
In a regexp, a backslash before any character that is not in the above table
and not listed in
gawk-Specific Regexp Operators,
means that the next character should be taken literally, even if it would
normally be a regexp operator. For example, /a\+b/ matches the three
characters `a+b'.
For complete portability, do not use a backslash before any character not shown in the table above.
To summarize:
awk reads your program.
gawk processes both regexp constants and dynamic regexps
(see section Using Dynamic Regexps),
for the special operators listed in
gawk-Specific Regexp Operators.
If you place a backslash in a string constant before something that is
not one of the characters listed above, POSIX awk purposely
leaves what happens as undefined. There are two choices:
awk and gawk both do.
For example, "a\qc" is the same as "aqc".
(Because this is such an easy bug to both introduce and to miss,
gawk warns you about it.)
Consider `FS = "[ \t]+\|[ \t]+"' to use vertical bars
surrounded by whitespace as the field separator. There should be
two backslashes in the string, `FS = "[ \t]+\\|[ \t]+"'.)
awk implementations do this.
In such implementations, "a\qc" is the same as if you had typed
"a\\qc".
Suppose you use an octal or hexadecimal
escape to represent a regexp metacharacter
(see section Regular Expression Operators).
Does awk treat the character as a literal character or as a regexp
operator?
Historically, such characters were taken literally.
(d.c.)
However, the POSIX standard indicates that they should be treated
as real metacharacters, which is what gawk does.
In compatibility mode (see section Command-Line Options),
gawk treats the characters represented by octal and hexadecimal
escape sequences literally when used in regexp constants. Thus,
/a\52b/ is equivalent to /a\*b/.
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You can combine regular expressions with special characters, called regular expression operators or metacharacters, to increase the power and versatility of regular expressions.
The escape sequences described earlier in 3.2 Escape Sequences, are valid inside a regexp. They are introduced by a `\', and are recognized and converted into the corresponding real characters as the very first step in processing regexps.
Here is a list of metacharacters. All characters that are not escape sequences and that are not listed in the table stand for themselves:
\
^
It is important to realize that `^' does not match the beginning of a line embedded in a string. The condition is not true in the following example:
if ("line1\nLINE 2" ~ /^L/) ...
|
$
if ("line1\nLINE 2" ~ /1$/) ...
|
.
In strict POSIX mode (see section Command-Line Options),
`.' does not match the NUL
character, which is a character with all bits equal to zero.
Otherwise, NUL is just another character. Other versions of awk
may not be able to match the NUL character.
[...]
[^ ...]
|
The alternation applies to the largest possible regexps on either side.
(...)
*
The `*' repeats the smallest possible preceding expression. (Use parentheses if you want to repeat a larger expression.) It finds as many repetitions as possible. For example, `awk '/\(c[ad][ad]*r x\)/ { print }' sample' prints every record in `sample' containing a string of the form `(car x)', `(cdr x)', `(cadr x)', and so on. Notice the escaping of the parentheses by preceding them with backslashes.
+
awk '/\(c[ad]+r x\)/ { print }' sample
|
?
{n}
{n,}
{n,m}
wh{3}y
wh{3,5}y
wh{2,}y
Interval expressions were not traditionally available in awk.
They were added as part of the POSIX standard to make awk
and egrep consistent with each other.
However, because old programs may use `{' and `}' in regexp
constants, by default gawk does not match interval expressions
in regexps. If either `--posix' or `--re-interval' are specified
(see section Command-Line Options), then interval expressions
are allowed in regexps.
For new programs that use `{' and `}' in regexp constants,
it is good practice to always escape them with a backslash. Then the
regexp constants are valid and work the way you want them to, using
any version of awk.(13)
In regular expressions, the `*', `+', and `?' operators, as well as the braces `{' and `}', have the highest precedence, followed by concatenation, and finally by `|'. As in arithmetic, parentheses can change how operators are grouped.
In POSIX awk and gawk, the `*', `+', and `?' operators
stand for themselves when there is nothing in the regexp that precedes them.
For example, `/+/' matches a literal plus sign. However, many other versions of
awk treat such a usage as a syntax error.
If gawk is in compatibility mode
(see section Command-Line Options),
POSIX character classes and interval expressions are not available in
regular expressions.
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Within a character list, a range expression consists of two
characters separated by a hyphen. It matches any single character that
sorts between the two characters, using the locale's
collating sequence and character set. For example, in the default C
locale, `[a-dx-z]' is equivalent to `[abcdxyz]'. Many locales
sort characters in dictionary order, and in these locales,
`[a-dx-z]' is typically not equivalent to `[abcdxyz]'; instead it
might be equivalent to `[aBbCcDdxXyYz]', for example. To obtain
the traditional interpretation of bracket expressions, you can use the C
locale by setting the LC_ALL environment variable to the value
`C'.
To include one of the characters `\', `]', `-', or `^' in a character list, put a `\' in front of it. For example:
[d\]] |
matches either `d' or `]'.
This treatment of `\' in character lists
is compatible with other awk
implementations and is also mandated by POSIX.
The regular expressions in awk are a superset
of the POSIX specification for Extended Regular Expressions (EREs).
POSIX EREs are based on the regular expressions accepted by the
traditional egrep utility.
Character classes are a new feature introduced in the POSIX standard. A character class is a special notation for describing lists of characters that have a specific attribute, but the actual characters can vary from country to country and/or from character set to character set. For example, the notion of what is an alphabetic character differs between the United States and France.
A character class is only valid in a regexp inside the brackets of a character list. Character classes consist of `[:', a keyword denoting the class, and `:]'. Here are the character classes defined by the POSIX standard:
[:alnum:] | Alphanumeric characters. |
[:alpha:] | Alphabetic characters. |
[:blank:] | Space and tab characters. |
[:cntrl:] | Control characters. |
[:digit:] | Numeric characters. |
[:graph:] | Characters that are both printable and visible. | (A space is printable but not visible, whereas an `a' is both.)
[:lower:] | Lowercase alphabetic characters. |
[:print:] | Printable characters (characters that are not control characters). |
[:punct:] | Punctuation characters (characters that are not letters, digits, | control characters, or space characters).
[:space:] | Space characters (such as space, tab, and formfeed, to name a few). |
[:upper:] | Uppercase alphabetic characters. |
[:xdigit:] | Characters that are hexadecimal digits. |
For example, before the POSIX standard, you had to write /[A-Za-z0-9]/
to match alphanumeric characters. If your
character set had other alphabetic characters in it, this would not
match them, and if your character set collated differently from
ASCII, this might not even match the ASCII alphanumeric characters.
With the POSIX character classes, you can write
/[[:alnum:]]/ to match the alphabetic
and numeric characters in your character set.
Two additional special sequences can appear in character lists. These apply to non-ASCII character sets, which can have single symbols (called collating elements) that are represented with more than one character. They can also have several characters that are equivalent for collating, or sorting, purposes. (For example, in French, a plain "e" and a grave-accented "è" are equivalent.)
[[.ch.]] is a regexp that matches this collating element, whereas
[ch] is a regexp that matches either `c' or `h'.
[[=e=]] is a regexp
that matches any of `e', `é', or `è'.
These features are very valuable in non-English speaking locales.
Caution: The library functions that gawk uses for regular
expression matching currently only recognize POSIX character classes;
they do not recognize collating symbols or equivalence classes.
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gawk-Specific Regexp Operators
GNU software that deals with regular expressions provides a number of
additional regexp operators. These operators are described in this
section and are specific to gawk;
they are not available in other awk implementations.
Most of the additional operators deal with word matching.
For our purposes, a word is a sequence of one or more letters, digits,
or underscores (`_'):
\w
[[:alnum:]_].
\W
[^[:alnum:]_].
\<
/\<away/ matches `away' but not
`stowaway'.
\>
/stow\>/ matches `stow' but not `stowaway'.
\y
\B
/\Brat\B/ matches `crate' but it does not match `dirty rat'.
`\B' is essentially the opposite of `\y'.
There are two other operators that work on buffers. In Emacs, a
buffer is, naturally, an Emacs buffer. For other programs,
gawk's regexp library routines consider the entire
string to match as the buffer.
\`
\'
Because `^' and `$' always work in terms of the beginning
and end of strings, these operators don't add any new capabilities
for awk. They are provided for compatibility with other
GNU software.
In other GNU software, the word-boundary operator is `\b'. However,
that conflicts with the awk language's definition of `\b'
as backspace, so gawk uses a different letter.
An alternative method would have been to require two backslashes in the
GNU operators, but this was deemed too confusing. The current
method of using `\y' for the GNU `\b' appears to be the
lesser of two evils.
The various command-line options
(see section Command-Line Options)
control how gawk interprets characters in regexps:
gawk provides all the facilities of
POSIX regexps and the
previously described
GNU regexp operators.
GNU regexp operators described
in Regular Expression Operators.
However, interval expressions are not supported.
--posix
--traditional
awk regexps are matched. The GNU operators
are not special, interval expressions are not available, nor
are the POSIX character classes ([[:alnum:]] and so on).
Characters described by octal and hexadecimal escape sequences are
treated literally, even if they represent regexp metacharacters.
--re-interval
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Case is normally significant in regular expressions, both when matching ordinary characters (i.e., not metacharacters) and inside character sets. Thus, a `w' in a regular expression matches only a lowercase `w' and not an uppercase `W'.
The simplest way to do a case-independent match is to use a character list--for example, `[Ww]'. However, this can be cumbersome if you need to use it often and it can make the regular expressions harder to read. There are two alternatives that you might prefer.
One way to perform a case-insensitive match at a particular point in the
program is to convert the data to a single case, using the
tolower or toupper built-in string functions (which we
haven't discussed yet;
see section String Manipulation Functions).
For example:
tolower($1) ~ /foo/ { ... }
|
converts the first field to lowercase before matching against it.
This works in any POSIX-compliant awk.
Another method, specific to gawk, is to set the variable
IGNORECASE to a nonzero value (see section 7.5 Built-in Variables).
When IGNORECASE is not zero, all regexp and string
operations ignore case. Changing the value of
IGNORECASE dynamically controls the case sensitivity of the
program as it runs. Case is significant by default because
IGNORECASE (like most variables) is initialized to zero:
x = "aB" if (x ~ /ab/) ... # this test will fail IGNORECASE = 1 if (x ~ /ab/) ... # now it will succeed |
In general, you cannot use IGNORECASE to make certain rules
case-insensitive and other rules case-sensitive, because there is no
straightforward way
to set IGNORECASE just for the pattern of
a particular rule.(14)
To do this, use either character lists or tolower. However, one
thing you can do with IGNORECASE only is dynamically turn
case-sensitivity on or off for all the rules at once.
IGNORECASE can be set on the command line or in a BEGIN rule
(see section Other Command-Line Arguments; also
see section Startup and Cleanup Actions).
Setting IGNORECASE from the command line is a way to make
a program case-insensitive without having to edit it.
Prior to gawk 3.0, the value of IGNORECASE
affected regexp operations only. It did not affect string comparison
with `==', `!=', and so on.
Beginning with version 3.0, both regexp and string comparison
operations are also affected by IGNORECASE.
Beginning with gawk 3.0,
the equivalences between upper-
and lowercase characters are based on the ISO-8859-1 (ISO Latin-1)
character set. This character set is a superset of the traditional 128
ASCII characters, that also provides a number of characters suitable
for use with European languages.
The value of IGNORECASE has no effect if gawk is in
compatibility mode (see section Command-Line Options).
Case is always significant in compatibility mode.
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echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'
|
This example uses the sub function (which we haven't discussed yet;
see section String Manipulation Functions)
to make a change to the input record. Here, the regexp /a+/
indicates "one or more `a' characters," and the replacement
text is `<A>'.
The input contains four `a' characters.
awk (and POSIX) regular expressions always match
the leftmost, longest sequence of input characters that can
match. Thus, all four `a' characters are
replaced with `<A>' in this example:
$ echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'
-| <A>bcd
|
For simple match/no-match tests, this is not so important. But when doing
text matching and substitutions with the match, sub, gsub,
and gensub functions, it is very important.
See section String Manipulation Functions,
for more information on these functions.
Understanding this principle is also important for regexp-based record
and field splitting (see section How Input Is Split into Records,
and also see section Specifying How Fields Are Separated).
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The righthand side of a `~' or `!~' operator need not be a regexp constant (i.e., a string of characters between slashes). It may be any expression. The expression is evaluated and converted to a string if necessary; the contents of the string are used as the regexp. A regexp that is computed in this way is called a dynamic regexp:
BEGIN { digits_regexp = "[[:digit:]]+" }
$0 ~ digits_regexp { print }
|
This sets digits_regexp to a regexp that describes one or more digits,
and tests whether the input record matches this regexp.
When using the `~' and `!~'
Caution: When using the `~' and `!~'
operators, there is a difference between a regexp constant
enclosed in slashes and a string constant enclosed in double quotes.
If you are going to use a string constant, you have to understand that
the string is, in essence, scanned twice: the first time when
awk reads your program, and the second time when it goes to
match the string on the lefthand side of the operator with the pattern
on the right. This is true of any string valued expression (such as
digits_regexp shown previously), not just string constants.
What difference does it make if the string is scanned twice? The answer has to do with escape sequences, and particularly with backslashes. To get a backslash into a regular expression inside a string, you have to type two backslashes.
For example, /\*/ is a regexp constant for a literal `*'.
Only one backslash is needed. To do the same thing with a string,
you have to type "\\*". The first backslash escapes the
second one so that the string actually contains the
two characters `\' and `*'.
Given that you can use both regexp and string constants to describe regular expressions, which should you use? The answer is "regexp constants," for several reasons:
awk can note
that you have supplied a regexp, and store it internally in a form that
makes pattern matching more efficient. When using a string constant,
awk must first convert the string into this internal form and
then perform the pattern matching.
\n in Character Lists of Dynamic Regexps
Some commercial versions of awk do not allow the newline
character to be used inside a character list for a dynamic regexp:
$ awk '$0 ~ "[ \t\n]"' error--> awk: newline in character class [ error--> ]... error--> source line number 1 error--> context is error--> >>> <<< |
But a newline in a regexp constant works with no problem:
$ awk '$0 ~ /[ \t\n]/' here is a sample line -| here is a sample line Ctrl-d |
gawk does not have this problem, and it isn't likely to
occur often in practice, but it's worth noting for future reference.
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