Common mistakes

Common mistakes

The most common mistakes when working with regular expressions are collected in Table 2.7.

Table 2.7. Common mistakes when working with regular expressions

Problem	Cause	Fix
an IsMatch check accepts a string with extra characters	a pattern without anchors looks for a match anywhere	^…$ or ^…\z
a string with a trailing \n passes validation	$ also matches the position before \n	\z instead of $
"\d+" does not compile (CS1009)	\d in a regular C# string is an escape sequence	@"\d+" or a raw string
.* captures too much	a greedy quantifier	.*? or a negated class [^<]*
[А-Яа-я] does not accept "Їжак"	the Ukrainian letters Ґ, Є, І, Ї are outside the range	\p{IsCyrillic} or a full list of letters
\d accepts Arabic-Indic digits	\d means any Unicode digit	[0-9] or RegexOptions.ECMAScript
RegexParseException for user input	metacharacters +, (, . in the input	Regex.Escape
the program "hangs" on some strings	nested quantifiers, catastrophic backtracking	a simpler pattern, matchTimeout, NonBacktracking