What is the meaning of (^| )ABC( |$) as an extended REGEX ? I came here across the command **grep -E '(^| )ABC( |$)' file1** I am confused how to interpret the regular expression therein, because I know the ^ and $ character as beginning and end signifier, but in here they are not at the very beginning or end because of the bracket, ^ is sometimes used to negate a set of character, but I have seen this usage only in [^....] like expression. Could someone explain step by step the meaning of this regex ?

It means that `grep` should search for `ABC` string only at the beginning of the line _OR_ after space, moreover this string has to end with another space OR the end of the line.

In other words someone wanted to search for a strings which form whole _words_. However this regexp has many issues, namely there could be many other characters before and after _word_ (at least in natural language), i.e. `(`,`)`,`.`,`;`,`:`,`,`,`...`, etc.

Thus, it is better to use `-w` option of `grep`, alternatively play with boundaries: `\b` or `\<`/`\>`.