81234567890, +71234567890, 1234567890 and 8 (123) 456-78-90 are only a few obvious formatting examples for Russian numbers. And if you're getting calls from multiple countries, the number of options is very high.
For a human, those options are not so different. All of them will work if you dial them; it's easy to compare them to understand that they are the same number. However, for software, they are completely different numbers. For example, if you have a contact saved in CRM with a number starting with +7, but you enter the number in the search with 8, then the search may not show this contact.^ - beginning of a line$ - end of a line. - any symbol\d - a digit\D - not a digit\w - a digit or a letter\W - not a digit and not a letter\s - space (including unusual varieties, such as no-line-break spaces)\S - not a space[ab\s0-5] - either: a, b, space or digit between 0 and 5abc 123 - literally "abc 123", possibly as a part of bigger text fragmentabc|123 - either "abc" or "123", possibly as a part of bigger text fragment^abc 123$ - "abc 123" as a whole line without additional text before or after\(, \), \[, \], \{, \}, \+, \?, \*, \., \|, \\ - literal representation of symbols that have special meaninng when not preceeded by \? - 0 or 1 time+ - 1 or more times* - any number of times{5} - 5 times{5,10} - 5 to 10 times{,10} - 0 to 10 times{5,} - 5 or more times\d{10} means "ten digits", \+? means "zero or one plus sign" and 0+ means "one or more zeroes".(10){3} means "101010".
Text from groups can be used in substitutions as $N where N is the number of a group.
For example, when using a pattern 8(\d{6})(\d{4}), the substitution $1 would mean the contents of the group (\d{6}) - the first 6 digits after "8". The substitution $2 would mean the contents of the group (\d{4}) - the following 4 digits.^\+?7(\d{10})$8$1+7 and 7 in the beginning of a number that consists of a prefix followed by 10 digits with 8, e.g. +74951234567 → 84951234567 and 74951234567 → 84951234567
This pattern is enabled by default^\+78+7 in the beginning of any number with 8, e.g. +712345671234567 → 812345671234567.*(\d{10})$8$18 and those 10 digits, e.g. 0071234567890 → 81234567890^(\d{7})$8495$18495 to a 7-digit number, e.g. 1234567 → 84951234567\D+7 (495) 123-45-67 → 74951234567[\s()-]+7 (495) 123-45-67 → +74951234567^(.*)(\d{3})(\d{3})(\d{2})(\d{2})$$1 ($2) $3-$4-$5x (xxx) xxx-xx-xx, e.g. +74951234567 → +7 (495) 123-45-67^0\d{2}(\d+)$8$18 and the digits from the end of a number, e.g. 0771234567890123 → 812345678901231234567$76543211234567 in the end of a number with 7654321, e.g. 84951234567 → 84957654321^84951234567$8495765432184951234567 with 84957654321.*(\d{4}$)$1+71234564321 → 4321^8123456(\d{4})$$18123456, leaving last 4 digits, e.g. 849576543214321 → 4321.*123456(\d{4})$$1123456 and 4 more digits, e.g. +71234564321 → 4321^1\d{3}$2$11 of a 4-digit number with 2, e.g. 1234 → 2234^1234$43211234 with 4321