php—PCRE regular expression conditional subgroup

You can make the matcher conditionally match a subgroup or select between two optional subgroups based on the result of an assertion or whether a previous capturing subgroup matches. The two syntaxes for conditional subgroups are as follows:

(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)

If the condition is met, use yes-pattern, otherwise use no-pattern (if specified). If there are more than 2 optional subgroups, a compile-time error will be generated.

There are two conditions. If the parentheses of (condition) are text consisting of numbers, the condition is satisfied when the (previous) subgroup represented by the number is matched (that is, using the yes-pattern). Consider the following pattern, which has some whitespace added to make it easier to read (see PCRE_EXTENDED option) and divided into three parts: ( ( )? [^()]+ (?(1) ) )

pattern The first part of matches an optional left parenthesis, and if that character occurs, sets it to the first subgroup of the captured substring. The second part matches one or more non-bracket characters. The third part is a conditional subgroup, which will test whether the first subgroup matches. If it matches, that is to say, the target string starts with a left bracket and the condition is TRUE, then use yes-pattern, which means that a match needs to be performed here. Right bracket. Otherwise, since no-pattern is not present, the subgroup does not match anything. In other words, this pattern matches a sequence of characters without parentheses or enclosed by closing parentheses.

If conditional string (R), it is satisfied when getting a recursive call to a pattern or subpattern. At the "top level", the condition is always false.

If the condition is not a sequence of numbers or (R), it must be an assertion. The assertion here can be arbitrary, positive, negative, forward, or backward. Consider this pattern, again with some whitespace added for readability, and two alternative paths on the second line.

(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

Conditional A positive assertion that matches an optional sequence of non-lowercase alphabetic characters followed by a lowercase letter. In other words, it tests for the presence of at least one lowercase letter in the target, and if a lowercase letter is found, the target matches the first optional branch, otherwise it matches the second branch. This pattern matches strings in two formats: dd-aaa-dd or dd-dd-dd. aaa represents lowercase letters and dd is a number.