Matches are Which was the first Sci-Fi story to predict obnoxious "robo calls"? I'd replace the. matched substring to be recalled, prefer non-capturing parentheses Asserts that what immediately follows the current position in the string is "check", Asserts that what immediately precedes the current position in the string is "check", Asserts that what immediately follows the current position in the string is not "check", Asserts that what immediately precedes the current position in the string is not "check". //split the string at the comma //assumes no commas in text $parts = preg_split ( '!,!', $string ); foreach ( $parts as $key => $value ) { //split the values at the = sign $parts [ $key ]= preg_split ( '!=!', $value ); foreach ( $parts [ $key] as $k2 => $v2 ) { //trim the quotes out and remove the slashes $parts [ $key ] [ $k2 ]= stripslashes ( )/.exec('3.141') Weighted sum of two random variables ranked by first order stochastic dominance. caret notation, where "X" is a letter from AZ (corresponding to codepoints It is explained in detail below in Advanced Searching With Flags. Substitutes the last group that was captured. Can I use the spell Immovable Object to create a castle which floats above the clouds? The link Espo provided was very helpful. All options are off by default. example, /\w/ matches "a" in "apple", "5" in "$5.28", and [abc] is functionally equivalent to (?:a|b|c). The match made with this part of the pattern is remembered for later use, as described in Using groups . Disjunctions are not atoms you need to use a group to make it part of a bigger pattern. For example, these regex will all match the exact same abc string : However in the third case, you've defined a capturing group which will enable you to access to b independently. Unfortunately syntax for case-insensitive matching is not common. How to replace string and preserve its uppercase/lowercase, Greedy vs. Lookahead assertion: Matches "x" only if "x" is behavior. For example, /\S\w*/ matches "foo" in "foo bar". You can specify options that control how the regular expression engine interprets a regular expression pattern. Is there such a thing as "right to be heard" by the authorities? For example, /(?