In many regex implementations, the following rules apply: Meta characters inside a character class are: ^ (negation) – (range) ] (end of the class) \ (escape char) So these should all be escaped. There are some corner cases though: – needs no escaping if placed at the very start, or end of the class ([abc-] … Read more
Perl-style regex patterns always need to be delimited. The very first character in the string is considered the delimiter, so something like this: function validate_email($email) { if (!preg_match(“/^[[:alnum:]][a-z0-9_.-]*@[a-z0-9.-]+\.[a-z]{2,4}$/i”, $email)) { echo ‘bad email’; } else { echo ‘good email’; } } The reason your initial attempt didn’t work is because it was trying to use … Read more
The ultimate answer to such a question is in the engine’s code, and at the bottom of the answer you’ll be able to dive into the section of the PCRE engine’s code responsible for ensuring fixed-length in lookbehinds—if you’re interested in knowing the finest details. In the meantime, let’s gradually zoom into the question from … Read more
Sqlite by default does not provide regex_replace function. You need to load it as an extension. Here is how i managed to do it. Download this C code for the extension (icu_replace) Compile it using gcc –shared -fPIC -I sqlite-autoconf-3071100 icu_replace.c -o icu_replace.so And in sqlite3 runn following command post above mentioned command has run … Read more
You need 4 backslashes to represent 1 in regex because: 2 backslashes are used for unescaping in a string (“\\\\” -> \\) 1 backslash is used for unescaping in the regex engine (\\ -> \) From the PHP doc, escaping any other character will result in the backslash being printed too1 Hence for \\\[, 1 … Read more
I know this isn’t exactly an answer to your question, but it’s helpful to have it here: Regular Expression to match valid XML Characters: [\u0009\u000a\u000d\u0020-\uD7FF\uE000-\uFFFD] So to remove invalid chars from XML, you’d do something like // filters control characters but allows only properly-formed surrogate sequences private static Regex _invalidXMLChars = new Regex( @”(?<![\uD800-\uDBFF])[\uDC00-\uDFFF]|[\uD800-\uDBFF](?![\uDC00-\uDFFF])|[\x00-\x08\x0B\x0C\x0E-\x1F\x7F-\x9F\uFEFF\uFFFE\uFFFF]”, RegexOptions.Compiled); … Read more
Lookahead and lookbehind aren’t nearly as similar as their names imply. The lookahead expression works exactly the same as it would if it were a standalone regex, except it’s anchored at the current match position and it doesn’t consume what it matches. Lookbehind is a whole different story. Starting at the current match position, it … Read more
To reuse a pattern, you could use (?n) where n is the number of the group to repeat. For example, your actual pattern : (PAT),(PAT), … ,(PAT) can be replaced by: (PAT),(?1), … ,(?1) (?1) is the same pattern as (PAT)whatever PAT is. You may have multiple patterns: (PAT1),(PAT2),(PAT1),(PAT2),(PAT1),(PAT2),(PAT1),(PAT2) may be reduced to: (PAT1),(PAT2),(?1),(?2),(?1),(?2),(?1),(?2) or: … Read more
PCRE and newlines PCRE has a superfluity of newline related escape sequences and alternatives. Well, a nifty escape sequence that you can use here is \R. By default \R will match Unicode newlines sequences, but it can be configured using different alternatives. To match any Unicode newline sequence that is in the ASCII range. preg_match(‘~\R~’, … Read more
Inspired by NullUserExceptions answer (which he already deleted as it failed for one case) I think I have found a solution myself: $regex = ‘~^ (?=(a(?-1)?b)c) a+(b(?-1)?c) $~x’; var_dump(preg_match($regex, ‘aabbcc’)); // 1 var_dump(preg_match($regex, ‘aaabbbccc’)); // 1 var_dump(preg_match($regex, ‘aaabbbcc’)); // 0 var_dump(preg_match($regex, ‘aaaccc’)); // 0 var_dump(preg_match($regex, ‘aabcc’)); // 0 var_dump(preg_match($regex, ‘abbcc’)); // 0 Try it yourself: … Read more