Why does expr match outputs different things when the pattern (not) contains a parenthesized group?

Why does

$ echo `expr match abcdef 'abc'`

give number of characters matched, which is 3, but

$ echo `expr match a开发者_运维百科bcdef '\(abc\)'`

gives the characters matched , which is abc ?

I understand regex matching is in play here, but cannot understand how having a parenthesized sub expression is making this difference here?

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This is from the man page of expr :

Pattern matches return the string matched between \( and \) or null; if \( and \) are not used, they return the number of characters matched or 0.

Man page.

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That has nothing to do with regular expressions. It is only a difference how the command "expr" works. The first one returns the length of the matching substring and the second one returns the matching substring itself.

There is a very good summary: TLDP refcard. You find a summary of combinations how expr can be used.