basic regex: [set], c* 0 or more c, cc* 1 or more c

extended regex: c+ 1 or more c

$ grep '^[a-fA-F][a-fA-F]*$' /usr/share/dict/words | wc -l
469
$ egrep '^[a-fA-F]+$' /usr/share/dict/words | wc -l
469
$ egrep '^[a-fA-F]+$' /usr/share/dict/words | grep def
def
deface
defaced
defade
$

---

sed 's/OLDregex/NEW/g'

sed 's;OLDregex;NEW;g'

sed 's/\//\\/g'  # replace / with \

sed 's;/;\\;g'


sed -e 's/onething/x/' -e 's/twothing/y/' ....

# bit flip:
$ echo 1010111110000 | sed -e 's/0/1/g' -e 's/1/0/g' # wrong
0000000000000
$ echo 1010111110000 | sed -e 's/0/Z/g' -e 's/1/0/g' -e 's/Z/1/g' # correct
0101000001111
$

x="abcabcabc"
y=$(echo "$x" | sed 's/a/Z/g') # replace all a's with Z's

---

$ ln -s /usr/share/dict/words .
$ grep ZZZZZ words
$ sed 's/^/ZZZZZ/' words > patterns

$ time grep -f patterns words

real    0m5.020s
user    0m3.240s
sys     0m1.077s

$ time fgrep -f patterns words

real    0m3.384s
user    0m3.190s
sys     0m0.167s

$ time cat patterns patterns | grep -f - words # double number of patterns, double time

real    0m10.077s
user    0m6.525s
sys     0m1.643s

$ time cat patterns patterns | fgrep -f - words # double number of patterns, ~same time

real    0m3.960s
user    0m3.762s
sys     0m0.185s
$

---

three times: last modified, changed (properties), used (accessed)

$ echo hi > j
$ cat j
hi
$ ls -l j
-rw-r--r-- 1 perry perry 3 Nov  8 05:11 j
# one minute later:
$ chmod 444 j
# one minute later:
$ cat j
hi
$ ls -l j
-r--r--r-- 1 perry perry 3 Nov  8 05:11 j
$ ls -lc j
-r--r--r-- 1 perry perry 3 Nov  8 05:12 j
$ ls -lu j
-r--r--r-- 1 perry perry 3 Nov  8 05:13 j
$
-----

$ find httpd-2.4.54 -name '*.ORIG' -print
httpd-2.4.54/include/ap_release.h.ORIG
httpd-2.4.54/modules/aaa/mod_authnz_ldap.c.ORIG
$ diff -c httpd-2.4.54/include/ap_release.h.ORIG httpd-2.4.54/include/ap_release.h
*** httpd-2.4.54/include/ap_release.h.ORIG	2022-06-06 10:20:09.000000000 -0400
--- httpd-2.4.54/include/ap_release.h	2022-11-02 09:57:33.013452448 -0400
***************
*** 39,45 ****
   */
  #define AP_SERVER_BASEVENDOR "Apache Software Foundation"
  #define AP_SERVER_BASEPROJECT "Apache HTTP Server"
! #define AP_SERVER_BASEPRODUCT "Apache"
  
  #define AP_SERVER_MAJORVERSION_NUMBER 2
  #define AP_SERVER_MINORVERSION_NUMBER 4
--- 39,45 ----
   */
  #define AP_SERVER_BASEVENDOR "Apache Software Foundation"
  #define AP_SERVER_BASEPROJECT "Apache HTTP Server"
! #define AP_SERVER_BASEPRODUCT "Chaos"
  
  #define AP_SERVER_MAJORVERSION_NUMBER 2
  #define AP_SERVER_MINORVERSION_NUMBER 4
$ diff -c httpd-2.4.54/modules/aaa/mod_authnz_ldap.c.ORIG httpd-2.4.54/modules/aaa/mod_authnz_ldap.c
*** httpd-2.4.54/modules/aaa/mod_authnz_ldap.c.ORIG	2021-03-02 02:43:35.000000000 -0500
--- httpd-2.4.54/modules/aaa/mod_authnz_ldap.c	2022-11-02 09:59:20.318489194 -0400
***************
*** 470,475 ****
--- 470,488 ----
  static authn_status authn_ldap_check_password(request_rec *r, const char *user,
                                                const char *password)
  {
+ //
+ // RP, 2020-01-20, remove @villanova.edu
+ //
+     if( user) {
+       char *p = strchr( user, '@');
+       if( p) {
+ 	char *q = apr_pstrdup(r->pool, user);
+ 	q[p-user] = 0;
+ 	user = q;
+ 	r->user = q;
+       }
+     }
+ 
      char filtbuf[FILTER_LENGTH];
      authn_ldap_config_t *sec =
          (authn_ldap_config_t *)ap_get_module_config(r->per_dir_config, &authnz_ldap_module);
$ nc -C www.ece.villanova.edu 80
HEAD / HTTP/1.0

HTTP/1.1 200 OK
Date: Tue, 08 Nov 2022 10:19:01 GMT
Server: Chaos
Last-Modified: Fri, 16 Jul 2021 10:34:13 GMT
ETag: "118-5c73b20f74082"
Accept-Ranges: bytes
Content-Length: 280
Connection: close
Content-Type: text/html; charset=UTF-8

^C
$ nc -C fog.misty.com 80
HEAD / HTTP/1.0

HTTP/1.1 200 OK
Date: Tue, 08 Nov 2022 10:19:18 GMT
Server: Apache/2.4.6 (CentOS) OpenSSL/1.0.2k-fips mod_fcgid/2.3.9 PHP/5.4.16
Last-Modified: Sun, 21 Nov 2021 12:07:53 GMT
ETag: "1456-5d14b5ba33d6e"
Accept-Ranges: bytes
Content-Length: 5206
Connection: close
Content-Type: text/html; charset=UTF-8

^C
$