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Conditional subpatterns

It is possible to cause the matching process to obey a subpattern conditionally or to choose between two alternative subpatterns, depending on the result of an assertion, or whether a previous capturing subpattern matched or not. The two possible forms of conditional subpattern are


If the condition is satisfied, the yes-pattern is used; otherwise the no-pattern (if present) is used. If there are more than two alternatives in the subpattern, a compile-time error occurs.

There are two kinds of condition. If the text between the parentheses consists of a sequence of digits, then the condition is satisfied if the capturing subpattern of that number has previously matched. Consider the following pattern, which contains non-significant white space to make it more readable (assume the PCRE_EXTENDED option) and to divide it into three parts for ease of discussion:

( \( )?    [^()]+    (?(1) \) )

The first part matches an optional opening parenthesis, and if that character is present, sets it as the first captured substring. The second part matches one or more characters that are not parentheses. The third part is a conditional subpattern that tests whether the first set of parentheses matched or not. If they did, that is, if subject started with an opening parenthesis, the condition is TRUE, and so the yes-pattern is executed and a closing parenthesis is required. Otherwise, since no-pattern is not present, the subpattern matches nothing. In other words, this pattern matches a sequence of non-parentheses, optionally enclosed in parentheses.

If the condition is the string (R), it is satisfied if a recursive call to the pattern or subpattern has been made. At "top level", the condition is false.

If the condition is not a sequence of digits or (R), it must be an assertion. This may be a positive or negative lookahead or lookbehind assertion. Consider this pattern, again containing non-significant white space, and with the two alternatives on the second line:

\d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

The condition is a positive lookahead assertion that matches an optional sequence of non-letters followed by a letter. In other words, it tests for the presence of at least one letter in the subject. If a letter is found, the subject is matched against the first alternative; otherwise it is matched against the second. This pattern matches strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.

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User Contributed Notes 2 notes

4gekkman at gmail dot com
11 months ago
In the Russian version of this page made ​​a serious mistake in the translation!

Here this line in the English version of the page:
"The condition is a positive lookahead assertion that matches an optional sequence of non-letters followed by a letter."

Here this line in the Russian version of the page:
"Приведен пример с утверждающим условием касательно предшествующего текста, которое выполняется для необязательной последовательности не-букв с последующей буквой."

Error in the translation of the word: "lookahead". It has been translated as: "предшествующего". But the correct translation is: "последующего".

Translator just made a mistake. He confused Look Ahead with Look Behind assertion.
3 years ago
Repetition of a subpattern will repeat conditionals that are contained inside it, updating subpattern matches with iteration.

Consider the following code, which scans thru HTML, keeping track of angle brackets "<" ">".  If open bracket "<" matches, then closing bracket ">" must follow before repetition can possibly end.  That way regex will effectively match only outside of tags.

'<html><body class=\'\'>\'\' went to '\'\meyer and ran
into <b>\'\'</b>.
echo preg_replace("%(*ANY)(.*?((<)(?(3).*?>).*?)*?)\'\'%s",'\1Fred',$subject);

Output will be:
'<html><body class=\'\'>Fred went to Fredmeyer and ran
into <b>Fred</b>.
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