Note that the numbering of the groups start with \1 and not with \0-a rare exception to the rule that in programming, all numbering starts with 0. You’ll use this feature a lot because it gives you much more expression power: for example, you can search for a name in a text-based on a given pattern and then process specifically this name in the rest of the text (and not all other names that would also fit the pattern). For example, to get the first matching group, you’d use the \1 special sequence.
In the first case, you simply get the first matching group with the \number special sequence.
#PYTHON REGEX GROUP HOW TO#
So the next question naturally arises: How to Get the First Matching Group? You can retrieve the matched content of each matching group. However, there’s a more advanced use of regex groups: retrieval. The use of the parentheses for structuring the regular expression is intuitive and should come naturally to you because the same rules apply as for arithmetic operations. Hence, the strings 'bacacaca', 'aaaa', '' (the empty string), and 'Xababababab' all match your regex. Say you create regex b?(a.)* with the matching group (a.) that matches all patterns starting with zero or one occurrence of character 'b' and an arbitrary number of two-character-sequences starting with the character 'a'. Let’s have a short example for the most basic use of a matching group-to structure the regex. You can retrieve it in other parts of the regular expression-or after analyzing the result of the whole regex matching. One big advantage of a matching group is that it captures the matched substring. And you can even have hierarchical matching groups, for example 'a(b|(cd))'. You can have multiple matching groups in a single regex. The whole content enclosed in the opening and closing parentheses is called matching group (or capture group). An example regex that does this is 'a(b|c)'.
Like you use parentheses to structure mathematical expressions, (2 + 2) * 2 versus 2 + (2 * 2), you use parentheses to structure regular expressions. Group Flags (?aiLmsux:…) and (?aiLmsux).Positive Lookahead Example: How to Match Two Words in Arbitrary Order?.
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