Why is `[\d]` ineffective in GNU `grep` when PCRE is enabled?

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In GNU grep, the regular expression [\d] matches the character class d only, because it does not have any special meaning in POSIX regular expressions.

However, when PCRE (Perl-Compatible Regular Expressions) is enabled, the backslash (\) is interpreted as an escape character for special characters, and the character class \d is recognized as shorthand for "digit character".

Therefore, in PCRE-enabled GNU grep, the correct syntax for matching digit characters would be \d or [[:digit:]].