Understanding The Regex pattern

Question

Can Anyone help me understand what the below pattern means:

/^([a-zA-Z0-9]){1}([a-zA-Z0-9\-\._]){0,27}\.([a-zA-Z]){1}([a-zA-Z0-9]){1,3}/

What a string must have so that it bounds to the above pattern.

These preg_match patterns are so confusing to understand. ! We are using it to see that a filename conforms to above pattern.

Regards, Sagar

Try feeding the expression to http://regex101.com — that'll get you a basic understanding. — Amal Murali, May 09 '14 at 07:56
[This visualization](http://i.stack.imgur.com/xXeDs.png) (from http://debuggex.com) might help as well. — Amal Murali, May 09 '14 at 07:58
Must start (`^`) with a single (`{1}`) alphanumeric character (`[a-zA-Z0-9]`), followed by up to 27 (`{0,27}`) further characters allowing alphanumeric, hyphen, dot or underscore (`[a-zA-Z0-9\-\._]`); a dot for the extension separator (`\.`); then an extension comprising a single (`{1}`) alpha character (`[a-zA-Z]`) and between 1 and 3 (`{1,3}`) alphanumeric characters (`[a-zA-Z0-9]`)..... it should really have a terminating `$` as well to indicate that this pattern must be a match against the entire string, not just the first part of it, otherwise `A.BC$%^&*` would match as valid — Mark Baker, May 09 '14 at 08:00

score 0 · Accepted Answer · answered May 09 '14 at 08:03

Basically, you are matching filenames that start with a letter or digit, followed by the characters letter, digit, -, ., _, any number of times up to 27, even 0 times. Then, there is a ., probably separating the extension, which is composed of a letter followed by the characters letter, digit any number of times from 1 to 3.

Some examples:

a.aa, 0_.A0a, A---____-_.a999

Understanding The Regex pattern

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