我们可以使用基于lookbehinds的条件句吗?

我有一个正则表达式,任务与此类似。这个想知道价格( demo ),其中美元符号必须在数字之前或之后,但决不能两者都在。

  \b       # "word" boundary
  (?<=\$)  # Assert that current position is preceded by $
  \d+\b    # digits, boundary
  (?!\$)   # Assert that current position is not followed by $
|          # alternation
  \b       # boundary
  (?<!\$)  # Assert that current position not preceded by $
  \d+\b    # digits, boundary
  (?=\$)   # Assert that current position is followed by $

PCRE中有没有一种方法可以使用一个条件来处理类似的条件。( demo )

(                 # capture 1, because we can't quantify lookarounds
  (?<=\$)         # Asserts that $ precedes current position
)?                # close capture 1, make it "optional" (0 or 1 times)

\b                # boundary
\d+               # digits
\b                # boundary
(?(1)             # conditional that capture #1 was a success
  (?!\$)          # if successful, assert that it is not followed by a #
|
  (?=\$)          # unsuccessful, assert that this position is followed by a $
)

注: 单词边界很重要,这样两者都可以捕获整个数字,否则regex将后退一步,从多个数字中剪切一个数字。

上面的两个表达式都匹配$15和$16$,但不匹配$17$。如果没有边界,它们将匹配15美元、16美元和17美元中的1美元。