01234
  s = 1FA7   big-endian

  x = 1F A7

       0  1
  x = A7 1F  little-endian <-


basic regex

 c  matches itself if not special
 \  escape
 \\ matches single \
 \c matches literal c if c is special
  .  matches any single char
 c* matches 0 or more c
 [ ] set
 ^  matches beginning of line
 $  matches end of line
 \( \) tag
 \1 \2 ...  back reference

extended regex

  |  or
 ( ) group


---
palindromes:

$ ln -s /usr/share/dict/words .
$ grep '....................' words | head
abdominohysterectomy
acetylcholinesterase
acetylmethylcarbinol
acetylphenylhydrazine
adrenocorticosteroid
adrenocorticotrophic
adrenocorticotrophin
aerobacteriologically
alkylbenzenesulfonate
alkylbenzenesulfonates
$ grep '^\(.\).\1$' words | head
AAA
aaa
ABA
aba
ADA
AEA
AFA
aga
AHA
aha
$ grep '^\(.\)\(.\).\2\1$' words | head
addda
ajaja
alala
alula
anana
arara
CAMAC
civic
deked
deled
$ grep '^\(.\)\(.\)\(.\)\3\2\1$' words | head
AAAAAA
degged
denned
hallah
kakkak
mallam
marram
pullup
redder
renner
$ grep '^\(.\)\(.\)\(.\).\3\2\1$' words | head
deedeed
deified
hagigah
halalah
murdrum
ottetto
peeweep
pip-pip
reifier
repaper
$