And these two regular expressions also match identical patterns:
One powerful option in creating search patterns is specifying that a subexpression that was matched earlier in a regular expression is matched again later in the expression. We do this using backreferences. Backreferences are named by the numbers 1 through 9, preceded by the backslash/escape character when used in this manner. These backreferences refer to each successive group in the match pattern, as in . Each numbered backreference refers to the group that, in this example, has the word corresponding to the number.
Regular Expression HOWTO — Python 3.6.4 …
Similar to regular parentheses, but the substring matched by the group isaccessible via the symbolic group name name. Group names must be validPython identifiers, and each group name must be defined only once within aregular expression. A symbolic group is also a numbered group, just as ifthe group were not named.
The solution is to use Python’s raw string notation for regular expressions;backslashes are not handled in any special way in a string literal prefixed with, so is a two-character string containing and ,while is a one-character string containing a newline. Regularexpressions will often be written in Python code using this raw string notation.
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Alternation, or the “or” operator. If A and B are regular expressions, will match any string that matches either A or B. has verylow precedence in order to make it work reasonably when you’re alternatingmulti-character strings. will match either or ,not , a or an , and .
Python regex: matching a parenthesis within parenthesis
Exception raised when a string passed to one of the functions here is not avalid regular expression (for example, it might contain unmatched parentheses)or when some other error occurs during compilation or matching. It is never anerror if a string contains no match for a pattern. The error instance hasthe following additional attributes:
Python - Literal parenthesis with python regex
The regular expression string matches strings that both begin and end with one or more numeric characters. Enter the following into the interactive shell:
Literal parenthesis with python regex
Another trick of advanced regular expression tools is "lookahead assertions." These are similar to regular grouped subexpression, except they do not actually grab what they match. There are two advantages to using lookahead assertions. On the one hand, a lookahead assertion can function in a similar way to a group that is not backreferenced; that is, you can match something without counting it in backreferences. More significantly, however, a lookahead assertion can specify that the next chunk of a pattern has a certain form, but let a different subexpression actually grab it (usually for purposes of backreferencing that other subexpression).
[Python] help with regex matching multiple %e - …
Scan through string looking for the first location where this regularexpression produces a match, and return a corresponding . Return if no position in the string matches thepattern; note that this is different from finding a zero-length match at somepoint in the string.