Use Apache 2.4's new generic expression parser to support setting uid/gid
dynamically, possibly based on the URL or other request-specific factors.
Here is a simple example with help from mod_rewrite:

  RewriteEngine on
  RewriteRule /~([a-z]+)/ - [E=ITKUID:$1]
  AssignUserIDExpr %{reqenv:ITKUID}

Index: httpd-2.4.2/server/mpm/itk/itk.c
===================================================================
--- httpd-2.4.2.orig/server/mpm/itk/itk.c
+++ httpd-2.4.2/server/mpm/itk/itk.c
@@ -41,6 +41,8 @@
 #if APR_HAVE_SYS_TYPES_H
 #include <sys/types.h>
 #endif
+#include <pwd.h>
+#include <grp.h>
 
 #include "ap_config.h"
 #include "httpd.h"
@@ -59,6 +61,7 @@
 #include "ap_listen.h"
 #include "ap_mmn.h"
 #include "apr_poll.h"
+#include "ap_expr.h"
 
 #include "seccomp.h"
 
@@ -181,6 +184,8 @@ typedef struct
     gid_t gid;
     char *username;
     int nice_value;
+    ap_expr_info_t *uid_expr;
+    ap_expr_info_t *gid_expr;
 } itk_per_dir_conf;
 
 typedef struct
@@ -1608,6 +1613,48 @@ static int itk_post_perdir_config(reques
         wanted_username = ap_unixd_config.user_name;
     }
 
+    /* AssignUserIDExpr and AssignGroupIDExpr override AssignUserID and defaults. */
+    if (dconf->uid_expr != NULL) {
+      struct passwd *ent;
+      const char *err;
+      wanted_username = ap_expr_str_exec(r, dconf->uid_expr, &err);
+      if (err) {
+        ap_log_error(APLOG_MARK, APLOG_ERR, 0, NULL, \
+            "Error while parsing AssignUserIDExpr expression: %s",
+            err);
+        return HTTP_INTERNAL_SERVER_ERROR;
+      }
+
+      if (!(ent = getpwnam(wanted_username))) {
+        ap_log_error(APLOG_MARK, APLOG_ERR, 0, NULL, \
+            "AssignUserIDExpr returned '%s', which is not a valid user name",
+            wanted_username);
+        return HTTP_INTERNAL_SERVER_ERROR;
+      }
+
+      wanted_uid = ent->pw_uid;
+    }
+    if (dconf->gid_expr != NULL) {
+      struct group *ent;
+      const char *err;
+      const char *wanted_groupname = ap_expr_str_exec(r, dconf->gid_expr, &err);
+      if (err) {
+        ap_log_error(APLOG_MARK, APLOG_ERR, 0, NULL, \
+            "Error while parsing AssignGroupIDExpr expression: %s",
+            err);
+        return HTTP_INTERNAL_SERVER_ERROR;
+      }
+
+      if (!(ent = getgrnam(wanted_groupname))) {
+        ap_log_error(APLOG_MARK, APLOG_ERR, 0, NULL, \
+            "AssignGroupIDExpr returned '%s', which is not a valid group name",
+            wanted_username);
+        return HTTP_INTERNAL_SERVER_ERROR;
+      }
+
+      wanted_gid = ent->gr_gid;
+    }
+
     if (!err && wanted_uid != -1 && wanted_gid != -1 && (getuid() != wanted_uid || getgid() != wanted_gid)) {
         if (setgid(wanted_gid)) {
             _DBG("setgid(%d): %s", wanted_gid, strerror(errno));
@@ -1802,6 +1849,53 @@ static const char *limit_gid_range(cmd_p
     return NULL;
 }
 
+static const char *assign_user_id_expr (cmd_parms *cmd, void *ptr, const char *user_name_expr)
+{
+    itk_per_dir_conf *dconf = (itk_per_dir_conf *) ptr;
+
+    const char *err;
+
+    err = ap_check_cmd_context(cmd, NOT_IN_HTACCESS);
+    if (err) {
+        return err;
+    }
+
+    dconf->uid_expr = ap_expr_parse_cmd_mi(cmd,
+                                           user_name_expr,
+                                           AP_EXPR_FLAG_STRING_RESULT,
+                                           &err,
+                                           NULL,
+                                           AP_CORE_MODULE_INDEX);
+    if (err) {
+        return err;
+    }
+
+    return NULL;
+}
+
+static const char *assign_group_id_expr (cmd_parms *cmd, void *ptr, const char *group_name_expr)
+{
+    itk_per_dir_conf *dconf = (itk_per_dir_conf *) ptr;
+
+    const char *err;
+
+    err = ap_check_cmd_context(cmd, NOT_IN_HTACCESS);
+    if (err) {
+        return err;
+    }
+
+    dconf->gid_expr = ap_expr_parse_cmd_mi(cmd,
+                                           group_name_expr,
+                                           AP_EXPR_FLAG_STRING_RESULT,
+                                           &err,
+                                           NULL,
+                                           AP_CORE_MODULE_INDEX);
+    if (err) {
+        return err;
+    }
+    return NULL;
+}
+
 static const char *set_max_clients_vhost (cmd_parms *cmd, void *dummy, const char *arg)
 {
     itk_server_conf *sconf =
@@ -1847,6 +1941,10 @@ AP_INIT_TAKE1("ServerLimit", set_server_
               "Maximum value of MaxRequestWorkers for this run of Apache"),
 AP_INIT_TAKE2("AssignUserID", assign_user_id, NULL, RSRC_CONF|ACCESS_CONF,
               "Tie a virtual host to a specific child process."),
+AP_INIT_RAW_ARGS("AssignUserIDExpr", assign_user_id_expr, NULL, RSRC_CONF|ACCESS_CONF,
+                 "Choose user ID given an expression. Will override AssignUserID."),
+AP_INIT_RAW_ARGS("AssignGroupIDExpr", assign_group_id_expr, NULL, RSRC_CONF|ACCESS_CONF,
+                 "Choose group ID given an expression. Will override AssignUserID."),
 AP_INIT_TAKE2("LimitUIDRange", limit_uid_range, NULL, RSRC_CONF,
               "If seccomp v2 is available (Linux 3.5.0+), limit the process's possible "
               "uid to the given range (inclusive endpoints)"),
@@ -1870,6 +1968,7 @@ static void *itk_create_dir_config(apr_p
     itk_per_dir_conf *c = (itk_per_dir_conf *)
         apr_pcalloc(p, sizeof(itk_per_dir_conf));
     c->uid = c->gid = -1;
+    c->uid_expr = c->gid_expr = NULL;
     c->nice_value = UNSET_NICE_VALUE;
     return c;
 }
@@ -1891,6 +1990,16 @@ static void *itk_merge_dir_config(apr_po
       c->uid = parent->uid;
       c->gid = parent->gid;
     }
+    if (child->uid_expr != NULL) {
+      c->uid_expr = child->uid_expr;
+    } else {
+      c->uid_expr = parent->uid_expr;
+    }
+    if (child->gid_expr != NULL) {
+      c->gid_expr = child->gid_expr;
+    } else {
+      c->gid_expr = parent->gid_expr;
+    }
     if (child->nice_value != UNSET_NICE_VALUE) {
       c->nice_value = child->nice_value;
     } else {