Regular Expressions 101

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/
(^\s*(?:\+?\d{1,3})?[-. (]*\d{3}[-. )]*\d{3}[-. ]*\d{4}(?: *x(\d+))?\s*)
/
gm
1st Capturing Group
(^\s*(?:\+?\d{1,3})?[-. (]*\d{3}[-. )]*\d{3}[-. ]*\d{4}(?: *x(\d+))?\s*)
^ asserts position at start of a line
\s
matches any whitespace character (equivalent to [\r\n\t\f\v ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Non-capturing group
(?:\+?\d{1,3})?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\+
matches the character + with index 4310 (2B16 or 538) literally (case sensitive)
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\d
matches a digit (equivalent to [0-9])
{1,3} matches the previous token between 1 and 3 times, as many times as possible, giving back as needed (greedy)
Match a single character present in the list below
[-. (]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
-. (
matches a single character in the list -. ( (case sensitive)
\d
matches a digit (equivalent to [0-9])
{3} matches the previous token exactly 3 times
Match a single character present in the list below
[-. )]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
-. )
matches a single character in the list -. ) (case sensitive)
\d
matches a digit (equivalent to [0-9])
{3} matches the previous token exactly 3 times
Match a single character present in the list below
[-. ]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
-.
matches a single character in the list -. (case sensitive)
\d
matches a digit (equivalent to [0-9])
{4} matches the previous token exactly 4 times
Non-capturing group
(?: *x(\d+))?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
matches the character with index 3210 (2016 or 408) literally (case sensitive)
x matches the character x with index 12010 (7816 or 1708) literally (case sensitive)
2nd Capturing Group
(\d+)
\d
matches a digit (equivalent to [0-9])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
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