regex101: PCRE (Legacy) Match Workspace

Regular Expression

^(?:[78]\d{10}|1\d{9}|0\d{6}(?:[87]\d|\d1)\d{9})$

New

85556987456
 -> starts with 8 and length is 11 -> match
75556987456
 -> starts with 7 and length is 11 -> match
1555698745
 -> starts with 1 and length is 10 -> match
000000085556987456
 -> starts with 0 and length is 18 and on pos 8 is a 8 -> match
000000075556987456
 -> starts with 0 and length is 18 and on pos 8 is a 7 -> match
000000001556987456
 -> starts with 0 and length is 18 and on pos 9 is a 1 -> match

the complete string are numbers AND
the first character is a 7 or 8 then the complete length of the string must be 11 OR
the first character is a 1 then the complete length of the string must be 10
OR the first character is a 0 then the complete length of the string must be 18 AND on character 8 must be a 8 or 7 OR on character 9 must be a 1

Substitution

85556987456
 -> starts with 8 and length is 11 -> match
75556987456
 -> starts with 7 and length is 11 -> match
1555698745
 -> starts with 1 and length is 10 -> match
000000085556987456
 -> starts with 0 and length is 18 and on pos 8 is a 8 -> match
000000075556987456
 -> starts with 0 and length is 18 and on pos 8 is a 7 -> match
000000001556987456
 -> starts with 0 and length is 18 and on pos 9 is a 1 -> match

the complete string are numbers AND
the first character is a 7 or 8 then the complete length of the string must be 11 OR
the first character is a 1 then the complete length of the string must be 10
OR the first character is a 0 then the complete length of the string must be 18 AND on character 8 must be a 8 or 7 OR on character 9 must be a 1