Regular Expressions 101

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/
^[+-]?(?!\.(?!\d)|$)\d*(?:\.\d*)?(?:(?<=[\d.])e[+-]?\d+)?$
/
igm
^ asserts position at start of a line
Match a single character present in the list below
[+-]
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
+-
matches a single character in the list +- (case insensitive)
Negative Lookahead
(?!\.(?!\d)|$)
Assert that the Regex below does not match
1st Alternative
\.(?!\d)
\. matches the character . with index 4610 (2E16 or 568) literally (case insensitive)
Negative Lookahead
(?!\d)
Assert that the Regex below does not match
\d matches a digit (equivalent to [0-9])
2nd Alternative
$
$ asserts position at the end of a line
\d
matches a digit (equivalent to [0-9])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Non-capturing group
(?:\.\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 4610 (2E16 or 568) literally (case insensitive)
\d
matches a digit (equivalent to [0-9])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Non-capturing group
(?:(?<=[\d.])e[+-]?\d+)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
Positive Lookbehind
(?<=[\d.])
Assert that the Regex below matches
Match a single character present in the list below
[\d.]
\d matches a digit (equivalent to [0-9])
. matches the character . with index 4610 (2E16 or 568) literally (case insensitive)
e matches the character e with index 10110 (6516 or 1458) literally (case insensitive)
Match a single character present in the list below
[+-]
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
+-
matches a single character in the list +- (case insensitive)
\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)
$ asserts position at the end of a line
Global pattern flags
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
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

/
/
igm

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