AWK - Regular Expressions 2

Regular Expressions

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 Operators   Operators 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    


3.1 How to Use Regular Expressions

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.


3.2 Escape Sequences

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 literal backslash, `\'.
\a
The "alert" character, Ctrl-g, ASCII code 7 (BEL). (This usually makes some sort of audible noise.)
\b
Backspace, Ctrl-h, ASCII code 8 (BS).
\f
Formfeed, Ctrl-l, ASCII code 12 (FF).
Newline, Ctrl-j, ASCII code 10 (LF).
Carriage return, Ctrl-m, ASCII code 13 (CR).
\t
Horizontal tab, Ctrl-i, ASCII code 9 (HT).
\v
Vertical tab, Ctrl-k, ASCII code 11 (VT).
\nnn
The octal value nnn, where nnn stands for 1 to 3 digits between `0' and `7'. For example, the code for the ASCII ESC (escape) character is `\033'.
\xhh...
The hexadecimal value hh, where hh stands for a sequence of hexadecimal digits (`0' through `9', and either `A' through `F' or `a' through `f'). Like the same construct in ISO C, the escape sequence continues until the first non-hexadecimal digit is seen. However, using more than two hexadecimal digits produces undefined results. (The `\x' escape sequence is not allowed in POSIX awk.)
\/
A literal slash (necessary for regexp constants only). This expression is used when you want to write a regexp constant that contains a slash. Because the regexp is delimited by slashes, you need to escape the slash that is part of the pattern, in order to tell awk to keep processing the rest of the regexp.
\"
A literal double quote (necessary for string constants only). This expression is used when you want to write a string constant that contains a double quote. Because the string is delimited by double quotes, you need to escape the quote that is part of the string, in order to tell 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:

 

  • The escape sequences in the table above are always processed first, for both string constants and regexp constants. This happens very early, as soon as 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.
  • A backslash before any other character means to treat that character literally.
Advanced Notes: Backslash Before Regular Characters

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:

Strip the backslash out
This is what Unix 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]+"'.)
Leave the backslash alone
Some other awk implementations do this. In such implementations, "a\qc" is the same as if you had typed "a\\qc".
Advanced Notes: Escape Sequences for Metacharacters

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/.


3.3 Regular Expression Operators

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:

 

\
This is used to suppress the special meaning of a character when matching. For example, `\$' matches the character `$'.
^
This matches the beginning of a string. For example, `^@chapter' matches `@chapter' at the beginning of a string, and can be used to identify chapter beginnings in Texinfo source files. The `^' is known as an anchor, because it anchors the pattern to match only at the beginning of the string.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/) ...
 
$
This is similar to `^' but it matches only at the end of a string. For example, `p$' matches a record that ends with a `p'. The `$' is an anchor and does not match the end of a line embedded in a string. The condition is not true in the following example:
 
if ("line1\nLINE 2" ~ /1$/) ...
 
.
This matches any single character, including the newline character. For example, `.P' matches any single character followed by a `P' in a string. Using concatenation, we can make a regular expression such as `U.A', that matches any three-character sequence that begins with `U' and ends with `A'. 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.

[...]
This is called a character list.(12) It matches any one of the characters that are enclosed in the square brackets. For example, `[MVX]' matches any one of the characters `M', `V', or `X', in a string. A full discussion of what can be inside the square brackets of a character list is given in Using Character Lists.
[^ ...]
This is a complemented character list. The first character after the `[' must be a `^'. It matches any characters except those in the square brackets. For example, `[^awk]' matches any character that is not an `a', a `w', or a `k'.
|
This is the alternation operator and it is used to specify alternatives. The `|' has the lowest precedence of all the regular expression operators. For example, `^P|[[:digit:]]' matches any string that matches either `^P' or `[[:digit:]]'. This means it matches any string that starts with `P' or contains a digit.The alternation applies to the largest possible regexps on either side.

(...)
Parentheses are used for grouping in regular expressions, similar to arithmetic. They can be used to concatenate regular expressions containing the alternation operator, `|'. For example, `@(samp|code)\{[^}]+\}' matches both `@code{foo}' and `@samp{bar}'. (These are Texinfo formatting control sequences.)
*
This symbol means that the preceding regular expression should be repeated as many times as necessary to find a match. For example, `ph*' applies the `*' symbol to the preceding `h' and looks for matches of one `p' followed by any number of `h's. This also matches just `p' if no `h's are present.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.

 
+
This symbol is similar to `*' except that the preceding expression must be matched at least once. This means that `wh+y' would match `why' and `whhy', but not `wy', whereas `wh*y' would match all three of these strings. The following is a simpler way of writing the last `*' example:
 
awk '/\(c[ad]+r x\)/ { print }' sample
 
?
This symbol is similar to `*' except that the preceding expression can be matched either once or not at all. For example, `fe?d' matches `fed' and `fd', but nothing else.
{n}
{n,}
{n,m}
One or two numbers inside braces denote an interval expression. If there is one number in the braces, the preceding regexp is repeated n times. If there are two numbers separated by a comma, the preceding regexp is repeated n to m times. If there is one number followed by a comma, then the preceding regexp is repeated at least n times:
wh{3}y
Matches `whhhy', but not `why' or `whhhhy'.
wh{3,5}y
Matches `whhhy', `whhhhy', or `whhhhhy', only.
wh{2,}y
Matches `whhy' or `whhhy', and so on.
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.


 

3.4 Using Character Lists

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:

(A space is printable but not visible, whereas an `a' is both.) control characters, or space characters).

[: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.
[:lower:] Lowercase alphabetic characters.
[:print:] Printable characters (characters that are not control characters).
[:punct:] Punctuation characters (characters that are not letters, digits,
[: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.)

 

Collating Symbols
A collating symbol is a multicharacter collating element enclosed between `[.' and `.]'. For example, if `ch' is a collating element, then [[.ch.]] is a regexp that matches this collating element, whereas [ch] is a regexp that matches either `c' or `h'.
Equivalence Classes
An equivalence class is a locale-specific name for a list of characters that are equal. The name is enclosed between `[=' and `=]'. For example, the name `e' might be used to represent all of "e," "è," and "é." In this case, [[=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.


 

3.5 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
Matches any word-constituent character--that is, it matches any letter, digit, or underscore. Think of it as short-hand for [[:alnum:]_].
\W
Matches any character that is not word-constituent. Think of it as short-hand for [^[:alnum:]_].
\<
Matches the empty string at the beginning of a word. For example, /\<away/ matches `away' but not `stowaway'.
\>
Matches the empty string at the end of a word. For example, /stow\>/ matches `stow' but not `stowaway'.
\y
Matches the empty string at either the beginning or the end of a word (i.e., the word boundary). For example, `\yballs?\y' matches either `ball' or `balls', as a separate word.
\B
Matches the empty string that occurs between two word-constituent characters. For example, /\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.

\`
Matches the empty string at the beginning of a buffer (string).
\'
Matches the empty string at the end of a buffer (string).

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:

 

No options
In the default case, 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
Only POSIX regexps are supported; the GNU operators are not special (e.g., `\w' matches a literal `w'). Interval expressions are allowed.
--traditional
Traditional Unix 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
Allow interval expressions in regexps, even if `--traditional' has been provided.


 

3.6 Case Sensitivity in Matching

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.


 

3.7 How Much Text Matches?

Consider the following:

 
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).


 

3.8 Using Dynamic Regexps

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:

 

  • String constants are more complicated to write and more difficult to read. Using regexp constants makes your programs less error-prone. Not understanding the difference between the two kinds of constants is a common source of errors.
  • It is more efficient to use regexp constants. 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.
  • Using regexp constants is better form; it shows clearly that you intend a regexp match.

Advanced Notes: Using 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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