Basic vs Extended (GNU Grep 3.7)

3.6 Basic vs Extended Regular Expressions

In basic regular expressions the characters ‘?’, ‘+’, ‘{’, ‘|’, ‘(’, and ‘)’ lose their special meaning; instead use the backslashed versions ‘\?’, ‘\+’, ‘\{’, ‘\|’, ‘$’, and ‘$’. Also, a backslash is needed before an interval expression’s closing ‘}’, and an unmatched \) is invalid.

Portable scripts should avoid the following constructs, as POSIX says they produce undefined results:

Extended regular expressions that use back-references.
Basic regular expressions that use ‘\?’, ‘\+’, or ‘\|’.
Empty parenthesized regular expressions like ‘()’.
Empty alternatives (as in, e.g, ‘a|’).
Repetition operators that immediately follow empty expressions, unescaped ‘$’, or other repetition operators.
A backslash escaping an ordinary character (e.g., ‘\S’), unless it is a back-reference.
An unescaped ‘[’ that is not part of a bracket expression.
In extended regular expressions, an unescaped ‘{’ that is not part of an interval expression.

Traditional egrep did not support interval expressions and some egrep implementations use ‘\{’ and ‘\}’ instead, so portable scripts should avoid interval expressions in ‘grep -E’ patterns and should use ‘[{]’ to match a literal ‘{’.

GNU grep -E attempts to support traditional usage by assuming that ‘{’ is not special if it would be the start of an invalid interval expression. For example, the command ‘grep -E '{1'’ searches for the two-character string ‘{1’ instead of reporting a syntax error in the regular expression. POSIX allows this behavior as an extension, but portable scripts should avoid it.