Why can't I put a regular expression on the left side of the ~ operator when using gawk?
For example, given the file below with fields delimited with tabs(\t):
$ cat cats siberian 1970 73 2500 shorthair 1999 60 3000 longhair 1998 102 9859 scottish 2001 30 6000
If I use gawk to find a record, it works:
$ gawk '$1 ~ /h/' cats shorthair 1999 60 3000 longhair 1998 102 9859 scottish 2001 30 6000
However if I move the operands $1 and /h/ around, it doesn't:
$ gawk '/h/ ~ $1' cats gawk: cmd. line:1: warning: regular expression on left of `~' or `!~' operator
The gawk man page for the ~ operator says:
Regular expression match, negated match. NOTE: Do not use a constant regular expression (/foo/) on the left-hand side of a ~ or !~. Only use one on the right-hand side. The expression /foo/ ~ exp has the same meaning as (($0 ~ /foo/) ~ exp). This is usually not what was intended.
I don't understand how the expression /foo/ is evaluated to become ($0 ~ /foo/) and also this seems to only imply the weaker phrase "bad things will happen if you put a constant regular expression on the left" it doesn't actually imply the stronger phrase of "the behaviour of gawk is undefined if you put a constant regular expression on the left because it wasn't programmed to be used in this way".
I basically don't understand how the operator ~ is evaluated internally.