<regex>: Limit backreference parsing to single digit for basic regular expressions
#5167
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According to Sections 9.3.6 and 9.5.1 of the POSIX standard, a backreference is a backslash followed by a single digit, so
\1to\9. Thus,\(a\)\10is a valid basic regular expression matching the stringaa0, because\10is a backreference to capture group 1 (\(a\)) plus the digit 0. However, the regex parser currently rejects this regular expression because it interprets\10as a backreference to capture group 10.After this PR, the parser only reads a single digit in basic regular expression (and grep) mode, so
\10is parsed correctly as a backreference to capture group 1 plus digit 0.