396 lines
6.4 KiB
HTML
396 lines
6.4 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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><TITLE
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>Regular expressions</TITLE
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NAME="GENERATOR"
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>Bash Guide for Beginners</TH
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>Chapter 4. Regular expressions</TD
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="sect1"
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><H1
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CLASS="sect1"
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><A
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NAME="sect_04_01"
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></A
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>4.1. Regular expressions</H1
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="sect_04_01_01"
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></A
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>4.1.1. What are regular expressions?</H2
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><P
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>A <EM
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>regular expression</EM
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> is a pattern that describes a set of strings. Regular expressions are constructed analogously to arithmetic expressions by using various operators to combine smaller expressions.</P
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><P
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>The fundamental building blocks are the regular expressions that match a single character. Most characters, including all letters and digits, are regular expressions that match themselves. Any metacharacter with special meaning may be quoted by preceding it with a backslash.</P
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></DIV
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="sect_04_01_02"
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></A
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>4.1.2. Regular expression metacharacters</H2
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><P
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>A regular expression may be followed by one of several repetition operators (metacharacters):</P
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><DIV
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CLASS="table"
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><A
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NAME="table_04_01"
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></A
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><P
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><B
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>Table 4-1. Regular expression operators</B
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></P
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><TABLE
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BORDER="1"
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CLASS="CALSTABLE"
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><THEAD
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><TR
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><TH
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Operator</TH
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><TH
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Effect</TH
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></TR
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></THEAD
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><TBODY
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>.</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Matches any single character.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>?</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>The preceding item is optional and will be matched, at most, once.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>*</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>The preceding item will be matched zero or more times.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>+</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>The preceding item will be matched one or more times.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>{N}</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>The preceding item is matched exactly N times.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>{N,}</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>The preceding item is matched N or more times.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>{N,M}</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>The preceding item is matched at least N times, but not more than M times.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>-</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>represents the range if it's not first or last in a list or the ending point of a range in a list.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>^</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Matches the empty string at the beginning of a line; also represents the characters not in the range of a list.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>$</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Matches the empty string at the end of a line.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>\b</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Matches the empty string at the edge of a word.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>\B</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Matches the empty string provided it's not at the edge of a word.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>\<</TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Match the empty string at the beginning of word.</TD
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></TR
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>\></TD
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><TD
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ALIGN="LEFT"
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VALIGN="MIDDLE"
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>Match the empty string at the end of word.</TD
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></TR
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></TBODY
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></TABLE
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></DIV
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><P
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>Two regular expressions may be concatenated; the resulting regular expression matches any string formed by concatenating two substrings that respectively match the concatenated subexpressions.</P
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><P
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>Two regular expressions may be joined by the infix operator <SPAN
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CLASS="QUOTE"
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>"|"</SPAN
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>; the resulting regular expression matches any string matching either subexpression.</P
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><P
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>Repetition takes precedence over concatenation, which in turn takes precedence over alternation. A whole subexpression may be enclosed in parentheses to override these precedence rules.</P
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></DIV
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="sect_04_01_03"
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></A
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>4.1.3. Basic versus extended regular expressions</H2
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><P
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>In basic regular expressions the metacharacters <SPAN
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CLASS="QUOTE"
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>"?"</SPAN
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>, <SPAN
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CLASS="QUOTE"
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>"+"</SPAN
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>, <SPAN
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CLASS="QUOTE"
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>"{"</SPAN
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>, <SPAN
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CLASS="QUOTE"
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>"|"</SPAN
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>, <SPAN
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CLASS="QUOTE"
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>"("</SPAN
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>, and <SPAN
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CLASS="QUOTE"
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>")"</SPAN
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> lose their special meaning; instead use the backslashed versions <SPAN
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CLASS="QUOTE"
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>"\?"</SPAN
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>, <SPAN
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CLASS="QUOTE"
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>"\+"</SPAN
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>, <SPAN
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CLASS="QUOTE"
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>"\{"</SPAN
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>, <SPAN
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CLASS="QUOTE"
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>"\|"</SPAN
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>, <SPAN
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CLASS="QUOTE"
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>"\("</SPAN
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>, and <SPAN
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CLASS="QUOTE"
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>"\)"</SPAN
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>.</P
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><P
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>Check in your system documentation whether commands using regular expressions support extended expressions.</P
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></DIV
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>Regular expressions</TD
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>Examples using grep</TD
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