Shell Programming and Scripting

I am an amateur in bash scripting and RE. Could someone please tel me in precise English, what is the meaning of the following:

Code:

regex='^([1-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(\.([0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3}$'

Thanks

Code:

^([1-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])((\.([0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3}$

There are 4 major sections. The red '^' at the beginning 'anchors' the pattern to the beginning of the data. The expression will only evaluate to true if the first character of the pattern is the first character of the data.

The red '$' at the end is also an anchor which anchors the match to the end of the data. The expression will evaluate to true only if the last character of the matched string is the last character of the data.

There are two major sections in between (blue and black). The blue section consists of a group which is contained within parens. Within the group are pipe '|' separated fields; the pipe indicates an OR condition such that the pattern is considered a match when any single field's expression is true.

The blue group contains a set of 4 fields which will evaluate to true when the chararacters at the beginning
of the string match:

Code:

 [1-9]           one number 0-9
OR
 [1-9][0-9]     two numbers one number in the set 1-9 followed by a number in the set 0-9
OR
 1[0-9][0-9]    three numbers 1 followed by a number in the set 0-9 followed by a number in the set 0-9
OR
 2[0-4][0-9]    three numbers 2 followed by a number in the set 0-4 followed by a number in the set 0-9
OR
  25[0-5]        three numbers 2 then 5 followed by a number in the set 0-5

The second section (black) contains a nested group. The inner group is similar to the blue group. The first number is allowed to be 0 even if a single digit, and the set of up to three digits must be preceded with a dot. There is a backslant (\) in front of the dot because the dot has special meaning and to indicate that a true (real) dot is to be matched it must be escaped.

The outer set of parens in the second group is followed by {3} which is a repetition operator that affects the adjacent group. The 3 indicates that this pattern must exist exactly three times.

I haven't completely vetted it, but it appears that the intent is to match an IP address.