gawk: Standard Regexp Constants
6.1.2.1 Standard Regular Expression Constants
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When used on the righthand side of the '~' or '!~' operators, a regexp
constant merely stands for the regexp that is to be matched. However,
regexp constants (such as '/foo/') may be used like simple expressions.
When a regexp constant appears by itself, it has the same meaning as if
it appeared in a pattern (i.e., '($0 ~ /foo/)'). (d.c.)
Expression Patterns. This means that the following two code segments:
if ($0 ~ /barfly/ || $0 ~ /camelot/)
print "found"
and:
if (/barfly/ || /camelot/)
print "found"
are exactly equivalent. One rather bizarre consequence of this rule is
that the following Boolean expression is valid, but does not do what its
author probably intended:
# Note that /foo/ is on the left of the ~
if (/foo/ ~ $1) print "found foo"
This code is "obviously" testing '$1' for a match against the regexp
'/foo/'. But in fact, the expression '/foo/ ~ $1' really means '($0 ~
/foo/) ~ $1'. In other words, first match the input record against the
regexp '/foo/'. The result is either zero or one, depending upon the
success or failure of the match. That result is then matched against
the first field in the record. Because it is unlikely that you would
ever really want to make this kind of test, 'gawk' issues a warning when
it sees this construct in a program. Another consequence of this rule
is that the assignment statement:
matches = /foo/
assigns either zero or one to the variable 'matches', depending upon the
contents of the current input record.
Constant regular expressions are also used as the first argument for
the 'gensub()', 'sub()', and 'gsub()' functions, as the second argument
of the 'match()' function, and as the third argument of the 'split()'
and 'patsplit()' functions (String Functions). Modern
implementations of 'awk', including 'gawk', allow the third argument of
'split()' to be a regexp constant, but some older implementations do
not. (d.c.) Because some built-in functions accept regexp constants as
arguments, confusion can arise when attempting to use regexp constants
as arguments to user-defined functions (User-defined). For
example:
function mysub(pat, repl, str, global)
{
if (global)
gsub(pat, repl, str)
else
sub(pat, repl, str)
return str
}
{
...
text = "hi! hi yourself!"
mysub(/hi/, "howdy", text, 1)
...
}
In this example, the programmer wants to pass a regexp constant to
the user-defined function 'mysub()', which in turn passes it on to
either 'sub()' or 'gsub()'. However, what really happens is that the
'pat' parameter is assigned a value of either one or zero, depending
upon whether or not '$0' matches '/hi/'. 'gawk' issues a warning when
it sees a regexp constant used as a parameter to a user-defined
function, because passing a truth value in this way is probably not what
was intended.