Regular Expressions 101

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/
^((\+7|\+8|7|8)*(9)+([0-9]){9})$
/
gm
^ asserts position at start of a line
1st Capturing Group
((\+7|\+8|7|8)*(9)+([0-9]){9})
2nd Capturing Group
(\+7|\+8|7|8)*
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data
1st Alternative
\+7
\+ matches the character + with index 4310 (2B16 or 538) literally (case sensitive)
7 matches the character 7 with index 5510 (3716 or 678) literally (case sensitive)
2nd Alternative
\+8
\+ matches the character + with index 4310 (2B16 or 538) literally (case sensitive)
8 matches the character 8 with index 5610 (3816 or 708) literally (case sensitive)
3rd Alternative
7
7 matches the character 7 with index 5510 (3716 or 678) literally (case sensitive)
4th Alternative
8
8 matches the character 8 with index 5610 (3816 or 708) literally (case sensitive)
3rd Capturing Group
(9)+
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data
9 matches the character 9 with index 5710 (3916 or 718) literally (case sensitive)
4th Capturing Group
([0-9]){9}
{9} matches the previous token exactly 9 times
A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data
Match a single character present in the list below
[0-9]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
$ asserts position at the end of a line
Global pattern flags
g modifier: global. All matches (don't return after first match)
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String