diff --git a/share/man/man7/d.7 b/share/man/man7/d.7
--- a/share/man/man7/d.7
+++ b/share/man/man7/d.7
@@ -198,6 +198,66 @@
 Suitable for timestamping logs.
 .El
 .Sh BUILT-IN FUNCTIONS
+.Bl -tag
+.It Ft string Fn strchr "string" "char"
+Return a substring of
+.Fa string
+starting at the first occurance of
+.Fa char
+in
+.Fa string .
+Similar to
+.Xr strchr 3 .
+.It Ft string Fn strjoin "string1" "string2"
+Return a string resulting from concatenating
+.Va string1
+and
+.Va string2 .
+Similar to
+.Xr strcat 3 .
+.It Ft string Fn strrchr "string" "char"
+Return a substring of
+.Va string
+starting at the last occurance of
+.Fa char
+in
+.Fa string .
+Similar to
+.Xr strrchr 3 .
+.It Ft string Fn strstr "haystack" "needle"
+Return a substring of
+.Fa haystack
+starting at the first occurrence of
+.Fa needle .
+Similar to
+.Xr strstr 3 .
+.It Ft string Fn strtok "string" "separators"
+Tokenize
+.Fa string
+with
+.Fa separators .
+Similar to
+.Xr strtok 3 .
+.It Ft size_t Fn strlen "string"
+Find the length of
+.Fa string .
+Similar to
+.Xr strlen 3 .
+.It Ft string Fn substr "string" "position" "[length]"
+Get a
+substring of
+.Fa string
+starting at
+.Fa position .
+The substring will be at most
+.Fa length Ns -long .
+If
+.Fa length
+is not specified, use the rest of the string.
+Similar to
+.Xr awk 1 Ap s
+.Fn substr .
+.El
 .Ss Aggregation Functions
 .Bl -tag -compact -width "llquantize(value, factor, low, high, nsteps)"
 .It Fn avg value