Regular Expressions 101

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@
(^[^<])|(<img[^>]>)|((?<=>).(?=<i))|((?<=>).$)
@
isug
1st Alternative
(^[^<])
1st Capturing Group
(^[^<])
^ asserts position at start of the string
Match a single character not present in the list below
[^<]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
< matches the character < literally (case insensitive)
2nd Alternative
(<img[^>]>)
2nd Capturing Group
(<img[^>]>)
<img matches the characters <img literally (case insensitive)
Match a single character not present in the list below
[^>]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
> matches the character > literally (case insensitive)
> matches the character > literally (case insensitive)
3rd Alternative
((?<=>).(?=<i))
3rd Capturing Group
((?<=>).(?=<i))
Positive Lookbehind
(?<=>)
Assert that the Regex below matches
> matches the character > literally (case insensitive)
.
matches any character, including unicode
+? matches the previous token between one and unlimited times, as few times as possible, expanding as needed (lazy)
Positive Lookahead
(?=<i)
Assert that the Regex below matches
<i matches the characters <i literally (case insensitive)
4th Alternative
((?<=>).$)
4th Capturing Group
((?<=>).$)
Positive Lookbehind
(?<=>)
Assert that the Regex below matches
> matches the character > literally (case insensitive)
.
matches any character, including unicode
+? matches the previous token between one and unlimited times, as few times as possible, expanding as needed (lazy)
$ asserts position at the end of the string, or before the line terminator right at the end of the string (if any)
Global pattern flags
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
s modifier: single line. Dot matches newline characters
u modifier: unicode. Pattern strings are treated as UTF-16. Also causes escape sequences to match unicode characters
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

@
@
isug

Test String

Code Generator

Generated Code

Loading code sample...
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