正则表达式匹配不超过一定数量的特定字符的行？

Question

This is my regex so far (assume PHP flavour):

^(([^\\\\]+)\\\\([^\\\\]+)){1,4}$

And my test data:

U:\16. New Products\#Complete\Bottle Openers\20170210 St Patrick Bottle Openers\Small Lifestyles
U:\16. New Products\#Complete\Canvas
U:\16. New Products

The goal is to find all lines with no more than 4 slashes. In this example I expect to match the second and third lines, however when I test that in regex101 it seems to match over multiple lines, despite having multiline set and using ^ and $ . What am I doing wrong?

Answer 1

The [^\\\\] pattern is a negated character class that matches any char but a \\ char, and thus, it can match line breaks. To quickly fix the issue, you might add \\n (and perhaps, \\r ) to the negated character class and use

^(([^\\\n\r]+)\\([^\\\n\r]+)){1,4}$

See the regex demo . The [^\\\\\\n\\r] cannot match CR and LF symbols and matches any char but a \\ , LF and CR chars.

A better regex for this task would be

^[^\\\n\r]*(?:\\[^\\\n\r]*){0,4}$

Or, with the last quantified part set to possessive to enhance efficiency:

^[^\\\n\r]*(?:\\[^\\\n\r]*){0,4}+$

See this regex demo .

Details

• ^ - start of string
• [^\\\\\\n\\r]* - zero or more chars other than \\ , LF and CR
• (?:\\\\[^\\\\\\n\\r]*){0,4} - 0 to 4 occurrences of
  • \\\\ - a \\ char
  • [^\\\\\\n\\r]* - zero or more chars other than \\ , LF and CR
• $ - end of string.