PHP Regular expression Observation points

the function of a regular expression separates, finds, matches, replaces the component separators of a string regular expression"/", "#", "~"General Atomic \d: 0-9 in decimal\d: Reverse \d: Except for 0-9\w:numbers, letters, bottom line \w:In addition to numbers, letters, line \s:spaces, line feeds, page breaks, carriage returns, character \s:Common Atomic characters in addition to spaces, line breaks, page breaks, carriage returns, and word-list characters. :any character other than a line break* :match the preceding content 0, 1, or more times? :Match Previous content 0 or 1 times^ :Opening dollar characters:End+ :match preceding content appears 1 or more times {n}:occurs n times {n,}: appears greater than or equal to n times {n, m}:occurs greater than or equal to n times less than or equal to M times []:Matches the [ABC] in the brackets to match A or B or C ():A back reference or as a whole [^]: take reverse [^ABC] except A, B, except C| :or [-] :represents the range pattern modifier i:Case insensitive m:Multi-text mode s:single-line mode U:Cancel greedy mode x:White space character A in the Ignore pattern:must start with this pattern D: If this modifier is set, the meta-character dollar sign in the pattern matches only the end of the target string. If this modifier is not set, when the string ends with a newline character, the dollar sign also matches the newline character (but does not match any previous newline character). If the modifier m is set,this modifier is ignored u: For utf-8 matching back reference$str= ' <b>abc</b> ';$pattern= '/<b> (. *) <\/b>/';Preg_replace($pattern, ' \\1 ',$str);//What \1 here represents is the match in parentheses, which is ABC .Greedy mode$str= ' <b>abc</b><b>bcd</b> ';//match every non-B tag$pattern= '/<b> (. *?) <\/b>/';//. * There is greedy mode, will always match to the last, with the question mark after the elimination of greedy mode, match one after the next matchPreg_replace($pattern, ' \\1 ',$str);//You can also cancel greedy mode by using the following methods$pattern= '/<b> (. *) <\/b>/u 'The regular expression Pcre function preg_filter-performs a regular expression search and replacePreg_grep-Returns an array entry for the matching pattern preg_last_error-returns the error code resulting from the last pcre regular executionPreg_match_all-Performs a global regular expression matchPreg_match-Perform a matching regular expressionPreg_quote-escaped regular expression character preg_replace_callback_array-perform a regular expression search and replace using callbacksPreg_replace_callback-Performs a regular expression search and replaces it with a callbackPreg_replace-Perform a search and replace of a regular expressionPreg_split-Write a regular expression by separating the string example with a regular expression to remove the SRC value from all the IMG tags in the page$str= '  ";$pattern= '//';/** * Analysis * img tag features: With  or >:/?> * Final stitching:  **/

PHP Regular expression Observation points