What are the differences between GNU grep's basic/extended and PCRE (`-P`) regular expressions?

Question

GNU grep's basic (BRE) and extended (ERE) syntax is documented at https://www.gnu.org/software/grep/manual/html_node/Regular-Expressions.html and PCRE is summarized at man pcresyntax, but there is no explicit comparison. What are the differences between GNU grep's basic/extended and PCRE (-P) regular expressions?

Answer 1

My research of the major syntax and functionality differences from http://www.greenend.org.uk/rjk/tech/regexp.html:

• . in GNU grep does not match null bytes and newlines (but does match newlines when used with --null-data), while Perl, everything except \n is matched.
• [...] in GNU grep defines POSIX bracket expressions, while Perl uses "character" classes. I'm not sure on the details. See http://www.greenend.org.uk/rjk/tech/regexp.html#bracketexpression
• "In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their special meaning; instead use the backslashed versions \?, \+, \{, \|, \(, and \)." From https://www.gnu.org/software/grep/manual/html_node/Basic-vs-Extended.html. ERE matches PCRE syntax.
• GNU grep \w and \W are the same as [[:alnum:]] and [^[:alnum]], while Perl uses alphanumeric and underscore.
• GNU grep has \< and \> for start and end of word.

Perl supports much more additional functionality:

• "nongreedy {}" with syntax re{...}?
• additional anchors and character types \A, \C, \d, \D, \G, \p, \P, \s, \S, \X. \Z, \z.
• (?#comment)
• shy grouping (?:re), shy grouping + modifiers (?modifiers:re)
• lookahead and negative lookahead (?=re) and (?!re), lookbehind and negative lookbehind (?<=p) and (?<!p)
• Atomic groups (?>re)
• Conditional expression (?(cond)re)
• ... and more, see man pcresyntax

For other engines, see Regular Expression Engine Comparison Chart by CMCDragonkai