Hello,
Awk seem treat the pattern as regular expression, how can awk search not using regular expression? e.g. [Aa] just represent for "[Aa]", not "A" or "a" . I don't want to add backslash [ and ] .

Hi,

The [ and ] are special and you do need to escape them (I.e. use backslashes).

This will search for [Aa] and not an A or an a: GNU Awk - 3.3 Regular Expression Operators

Hope this helps.

Thanks , I know to add backslash before special symbol, but I do not want to use so complex way. Does awk support a option to ignore these special symbol, and do not use regular expression?
e.g. I get a string from arguments, then search it in a file, the string may contain these special symbol, but add backslash before these special symbol is very complex, how to resolve it?

You can of course use substitution to change those metacharacters to prepend with slash. However, if you can use other programming language besides awk, here's one in Ruby

if you want to pass it from the shell

Thanks , I do want to use other languages. Do I must add backslash before the special symbols by using awk ? How to write a shell function to auto add backslash before the special symbol?

In awk, the index() function performs exact, non-regex, matches.

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915..., I cordially suggest to you that "the answer is, yes." Add the backslashes.

Awk is a well-built tool that is purpose-built for string matching and text-file manipulation. Regular expressions are one of its core strengths. Use those core strengths. There are compelling reasons why awk is built to work as it does, and there are many subtle strengths to reward your very diligent study of this tool.

You can also include a variable reference into a regular-expression, in which case awk is looking for "whatever this variable currently contains." (Set the variable's value to "[ab]".)

Notice that in this case the regular-expression is compiled to make a reference to the current content of the variable, so that the variable's current value is referenced each time the regular expression is evaluated, not when the regular-expression is compiled prior to its first use. (It is also possible to specify the pattern using a variable, i.e. so that the regular-expression definition is obtained from a variable's value, but that's another story for another day.)

Google this search-string, being mindful of the quotes: awk "regular expression" variable

Moved to Programming

I would be curious how you are using the string? Placing it in quotes opposite ~ or !~ is when it is used as
a computed regex (as described here), but when used opposite say == it will be just a string.