What is the difference between BRE and ERE?

Basic Regular Expressions (BRE) treat characters like the pipe, plus, question mark, braces and parentheses as literals unless backslash-escaped. Extended Regular Expressions (ERE) treat those same characters as metacharacters directly. BRE is the older POSIX form used by default in grep and sed.

Why does grep need escaped parentheses but grep -E does not?

Plain grep uses BRE, where parentheses are literal characters and grouping requires the escaped form . With grep -E it switches to ERE, where bare parentheses already mean grouping. This is the single most common source of confusion between the two flavors.

Can I use \d or \w in POSIX regex?

No. Shorthand classes like \d, \w and \s are Perl-compatible (PCRE) extensions and are not part of POSIX. The portable equivalents are bracket character classes such as [[:digit:]], [[:alnum:]] and [[:space:]] used inside a bracket expression.

Why must a character class be written as [[:digit:]]?

The token [:digit:] is only meaningful inside a bracket expression. The outer pair of brackets forms the bracket expression and the inner [:digit:] names the class, so a single digit is matched by [[:digit:]]. Writing [:digit:] alone matches the literal characters colon, d, i, g, i and t.

Do POSIX EREs support backreferences?

Strictly, no. POSIX defines backreferences (\1 through \9) only for Basic Regular Expressions. Many ERE engines add them as a non-portable extension, but relying on a backreference in awk or grep -E is not guaranteed by the standard.

POSIX Regex Reference

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POSIX defines two regular-expression dialects — Basic (BRE) and Extended (ERE) — and the difference between them trips up almost everyone who moves between grep, sed and awk. The same pattern can mean completely different things depending on which flavor a tool uses by default. This reference puts the two dialects side by side, lists the standard bracket character classes, and tells you which tool speaks which dialect.

How it works

In a Basic Regular Expression the special characters that do grouping, alternation and counting are written with a leading backslash: grouping is , alternation is \|, and a bound is \{m,n\}. Without the backslash those characters are literal. In an Extended Regular Expression the same constructs are written bare — ( ), |, {m,n} — and a backslash makes them literal instead. The anchors ^ and $, the dot ., the star *, and bracket expressions [...] behave the same in both dialects.

POSIX has no single-letter shorthand classes. Instead it provides named bracket character classes like [:alpha:], [:digit:] and [:space:], which must appear inside a bracket expression — so a digit is [[:digit:]]. This keeps matching correct across locales and character sets where the ASCII ranges alone would be wrong.

Tips and notes

Remember the tool defaults: grep and sed are BRE unless you pass -E, while awk and egrep are ERE. GNU adds convenience extensions (such as \+ and \? in BRE) that are not portable to other Unixes, so a script that must run anywhere should stick to the strict POSIX forms shown in the table.