Regular Expressions Special Matching Characters Revision as of 22:21, 6 February 2017 by ToddAdmin (Talk | contribs) (→‎Precedence) (diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)

The special symbols “{” and “}” group regular expressions for reference purposes. They permit the text matched by the expression within “{ }” to be referenced again in the search string or to be included as part of the replacement text.

An expression may contain up to 9 groups which are referenced by number, “\1” through “\9”. The groups are numbered in the order of their opening “{”.Groups may also be nested. Groups may be referenced in either a latter part of the regular expression or in the replacement string. This allows portions of the matched text to be used as parts of the replacement text.

The character “&” has a special meaning only in a replacement string and references the entire text matched by the search.

Consider the expression “the {man}|the {woman}”. If the matched text is “the man”, “\1” is “man” and “\2” is empty (null). Now consider the expression “{the {[a-z]+}} has”. If the matched text is “the woman has”, “\1” is “thewoman” and “\2” is “woman”.

{[a-z][a-z][a-z]}\1

Matches two contiguous occurrences of the same three letters. Will match “nownow”, “powpow”, etc.

{.*}\1

Matches any repeating text. Will match “nn”, “nownow”, “12341234”, etc.

^{.*}\1$

Matches a line consisting of two repeated occurrences of the same text (identical right and left halves).

^{.*}\N\1$

Matches two identical lines which occur together. (Can be used to find duplicate lines after sorting.)

Precedence

The order of precedence of the regular expressions operators is:

Highest > (to) Lowest \\ > [ ] > * +  ? > { } > Concatenations > |