regex: Add accessor to get max length of lookbehind characters

Use PCRE_INFO_MAXLOOKBEHIND to get the length (in characters) of the
longest lookbehind assertion in the pattern.

docs/reference/glib/glib-sections.txt

@@ -1004,6 +1004,7 @@ g_regex_ref
 g_regex_unref
 g_regex_get_pattern
 g_regex_get_max_backref
+g_regex_get_max_lookbehind
 g_regex_get_capture_count
 g_regex_get_has_cr_or_lf
 g_regex_get_string_number

glib/gregex.c

@@ -1525,6 +1525,34 @@ g_regex_get_max_backref (const GRegex *regex)
   return value;
 }
 
+
+/**
+ * g_regex_get_max_backref:
+ * @regex: a #GRegex
+ *
+ * Returns the lentgh of the longest lookbehind assertion in the pattern, in
+ * characters. Returns <literal>0</literal> if the pattern does not
+ * contain lookbehind assertions.
+ *
+ * This information may be useful when doing incremental partial matching; in
+ * that case you should always keep at least this many characters before the
+ * partially matched string.
+ *
+ * Returns: the length of the longest lookbehind assertion in the pattern
+ *
+ * Since: 2.34
+ */
+gint
+g_regex_get_max_lookbehind (const GRegex *regex)
+{
+  gint value;
+
+  pcre_fullinfo (regex->pcre_re, regex->extra,
+                 PCRE_INFO_MAXLOOKBEHIND, &value);
+
+  return value;
+}
+
 /**
  * g_regex_get_capture_count:
  * @regex: a #GRegex

glib/gregex.h

@@ -449,6 +449,7 @@ GRegex           *g_regex_ref			(GRegex              *regex);
 void		  g_regex_unref			(GRegex              *regex);
 const gchar	 *g_regex_get_pattern		(const GRegex        *regex);
 gint		  g_regex_get_max_backref	(const GRegex        *regex);
+gint              g_regex_get_max_lookbehind    (const GRegex        *regex);
 gint		  g_regex_get_capture_count	(const GRegex        *regex);
 gboolean          g_regex_get_has_cr_or_lf      (const GRegex        *regex);
 gint		  g_regex_get_string_number	(const GRegex        *regex,