^ : Start anchor
[0-9] : Character class to match one of the 10 digits
{1,6} : Range quantifier. Minimum 1 repetition and maximum 6.
$ : End anchor
\d can be used as an equivalent of [0-9], i.e. ^\d{1,6}$
a-zA-Z
?
. any char
\. the actual dot character
.? = .{0,1} match any char zero or one times
·* = .{0,} match any char zero or more times
.+ = .{1,} match any char one or more times
/=\/
[]=\[\]
-c, --count print only a count of matching lines per FILE
--color=auto highlight matching strings if (and only if) the output is written directly to the terminal and said terminal is capable of displaying colored output.
-C,--context show lines around
https://stackoverflow.com/questions/9081/grep-a-file-but-show-several-surrounding-lines
-e PATTERN, --regexp=PATTERN
-E, --extended-regexp
-n, --line-number print line number with output lines
-o, --only-matching
Print only the matched (non-empty) parts of a matching line,
with each such part on a separate output line.
-r, --recursive like --directories=recurse
Example: hightlight suffix
AccessControl___-1450542916.html
IP___1246962346.html
grep -Eor --color=auto "___[-]?[0-9]{10,10}"
https://ftp.gnu.org/old-gnu/Manuals/grep-2.4/html_node/grep_6.html
\w==[[:alnum:]]==[0-9A-Za-z][0-9A-Za-z]is dependent upon the ASCII character encoding, whereas[[:alnum:]]is portable.[:alnum:]Any of[:digit:]or[:alpha:]\W==[^[:alnum]]