Regular Expressions 101

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~
<a[^>]*>[^<]*</a>(*SKIP)(*FAIL)|https?:\S+
~
gm
1st Alternative
<a[^>]*>[^<]*</a>(*SKIP)(*FAIL)
<a
matches the characters <a literally (case sensitive)
Match a single character not 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 the character > with index 6210 (3E16 or 768) literally (case sensitive)
> matches the character > with index 6210 (3E16 or 768) literally (case sensitive)
Match a single character not 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 the character < with index 6010 (3C16 or 748) literally (case sensitive)
</a>
matches the characters </a> literally (case sensitive)
(*SKIP) acts like (*PRUNE), except that if the pattern is unanchored, the bumpalong advance is not to the next character, but to the position in the subject where (*SKIP) was encountered
(*FAIL) verb synonymous with (?!). Forces a matching failure at the given position in the pattern
2nd Alternative
https?:\S+
http
matches the characters http literally (case sensitive)
s
matches the character s with index 11510 (7316 or 1638) literally (case sensitive)
: matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
\S
matches any non-whitespace character (equivalent to [^\r\n\t\f\v ])
+ matches the previous token between one 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

Code Generator

Generated Code

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Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report.
For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php