diff --git a/reference/pcre/pattern.syntax.xml b/reference/pcre/pattern.syntax.xml
index fd6fc427fc..d4c4e66755 100644
--- a/reference/pcre/pattern.syntax.xml
+++ b/reference/pcre/pattern.syntax.xml
@@ -1040,6 +1040,48 @@
terminator is always special and must be escaped when used
within an expression.
+
+ Perl supports the POSIX notation for character classes. This uses names
+ enclosed by [: and :] within the enclosing square brackets. PCRE also
+ supports this notation. For example, [01[:alpha:]%]
+ matches "0", "1", any alphabetic character, or "%". The supported class
+ names are:
+
+ Character classes
+
+
+ alnumletters and digits
+ alphaletters
+ asciicharacter codes 0 - 127
+ blankspace or tab only
+ cntrlcontrol characters
+ digitdecimal digits (same as \d)
+ graphprinting characters, excluding space
+ lowerlower case letters
+ printprinting characters, including space
+ punctprinting characters, excluding letters and digits
+ spacewhite space (not quite the same as \s)
+ upperupper case letters
+ word"word" characters (same as \w)
+ xdigithexadecimal digits
+
+
+
+ The space characters are HT (9), LF (10), VT (11), FF (12), CR (13),
+ and space (32). Notice that this list includes the VT character (code
+ 11). This makes "space" different to \s, which does not include VT (for
+ Perl compatibility).
+
+
+ The name word is a Perl extension, and blank is a GNU extension
+ from Perl 5.8. Another Perl extension is negation, which is indicated
+ by a ^ character after the colon. For example,
+ [12[:^digit:]] matches "1", "2", or any non-digit.
+
+
+ In UTF-8 mode, characters with values greater than 128 do not match any
+ of the POSIX character classes.
+